Reflective optical stack for privacy display

ABSTRACT

A privacy display comprises a polarised output spatial light modulator, reflective polariser, plural polar control retarders and a polariser. In a privacy mode of operation, on-axis light from the spatial light modulator is directed without loss, whereas off-axis light has reduced luminance. Further, display reflectivity is reduced for on-axis reflections of ambient light, while reflectivity is increased for off-axis light. The visibility of the display to off-axis snoopers is reduced by means of luminance reduction and increased frontal reflectivity to ambient light. In a public mode of operation, the liquid crystal retardance is adjusted so that off-axis luminance and reflectivity are unmodified.

TECHNICAL FIELD

This disclosure generally relates to illumination from light modulationdevices, and more specifically relates to reflective optical stacks foruse in a display including a privacy display.

BACKGROUND

Privacy displays provide image visibility to a primary user that istypically in an on-axis position and reduced visibility of image contentto a snooper, that is typically in an off-axis position. A privacyfunction may be provided by micro-louvre optical films that transmit ahigh luminance from a display in an on-axis direction with low luminancein off-axis positions, however such films are not switchable, and thusthe display is limited to privacy only function.

Switchable privacy displays may be provided by control of the off-axisoptical output.

Control may be provided by means of luminance reduction, for example bymeans of switchable backlights for a liquid crystal display (LCD)spatial light modulator (SLM). Display backlights in general employwaveguides and light sources arranged along at least one input edge ofthe waveguide. Certain imaging directional backlights have theadditional capability of directing the illumination through a displaypanel into viewing windows. An imaging system may be formed betweenmultiple sources and the respective window images. One example of animaging directional backlight is an optical valve that may employ afolded optical system and hence may also be an example of a foldedimaging directional backlight. Light may propagate substantially withoutloss in one direction through the optical valve whilecounter-propagating light may be extracted by reflection off tiltedfacets as described in U.S. Pat. No. 9,519,153, which is hereinincorporated by reference in its entirety.

Control of off-axis privacy may further be provided by means of contrastreduction, for example by adjusting the liquid crystal bias tilt in anIn-Plane-Switching LCD.

BRIEF SUMMARY

According to a first aspect of the present disclosure there is provideda display device for use in ambient illumination comprising: a SLMarranged to output light; wherein the SLM comprises an output polariserarranged on the output side of the SLM, the output polariser being alinear polariser; an additional polariser arranged on the output side ofthe output polariser, the additional polariser being a linear polariser;a reflective polariser arranged between the output polariser and theadditional polariser, the reflective polariser being a linear polariser;and at least one polar control retarder arranged between the reflectivepolariser and the additional polariser, wherein the at least one polarcontrol retarder is capable of simultaneously introducing no netrelative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser along an axis along a normal to theplane of the at least one polar control retarder and introducing arelative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser along an axis inclined to a normal tothe plane of the at least one polar control retarder.

The at least one polar control retarder may be arranged to introduce nophase shift to polarisation components of light passed by the reflectivepolariser along an axis along a normal to the plane of the at least onepolar control retarder and/or to introduce a phase shift to polarisationcomponents of light passed by the reflective polariser along an axisinclined to a normal to the plane of the at least one polar controlretarder.

Advantageously a directional display may be provided which provides highreflectivity and low luminance for off-axis viewing positions; and lowreflectivity and high luminance for on-axis viewing positions. Suchincreased reflectivity and reduced luminance provides enhanced privacyperformance including increased visual security level (VSL) for off-axisviewers of the display in an ambiently illuminated environment. Aprivacy display may be provided with low visibility of images for anoff-axis snooper viewing the display in ambient conditions. The on-axisviewer may observe a substantially unmodified display. A low stray lightdisplay may be provided with low image visibility for some viewers andhigh image visibility for other viewers. The display may be used in anautomotive vehicle to prevent visibility to passengers or drivers.

The at least one polar control retarder may comprise a switchable liquidcrystal (LC) retarder comprising a layer of LC material, wherein the atleast one polar control retarder may be arranged, in a switchable stateof the switchable LC retarder, simultaneously to introduce no netrelative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser along an axis along a normal to theplane of the at least one polar control retarder and to introduce a netrelative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser along an axis inclined to a normal tothe plane of the at least one polar control retarder.

Advantageously a display may be switched between a privacy or low straymode with high reflectivity and low luminance to a snooper; and a wideviewing angle mode with increased luminance and reduced reflectivity foroff-axis users achieving high contrast images for multiple displayusers. The primary user may observe the display with substantially thesame high luminance and low reflectivity in both modes of operation.

The at least one polar control retarder may further comprise at leastone passive retarder which may be arranged to introduce no net relativephase shift to orthogonal polarisation components of light passed by thereflective polariser along an axis along a normal to the plane of the atleast one passive retarder and to introduce a net relative phase shiftto orthogonal polarisation components of light passed by the reflectivepolariser along an axis inclined to a normal to the plane of the atleast one passive retarder.

Advantageously the polar region over which high VSL may be achieved maybe substantially increased in comparison to displays with a switchableLC polar control retarder and no passive polar control retarders.

Where the at least one polar control retarder comprises a switchable LCretarder, in one alternative the switchable LC retarder may comprise twosurface alignment layers disposed adjacent to the LC material onopposite sides thereof and each arranged to provide homeotropicalignment at the adjacent LC material. The layer of LC material of theswitchable LC retarder may comprise a LC material with a negativedielectric anisotropy. The layer of LC material may have a retardancefor light of a wavelength of 550 nm in a range from 500 nm to 1000 nm,preferably in a range from 600 nm to 900 nm and most preferably in arange from 700 nm to 850 nm.

Where two surface alignment layers providing homeotropic alignment areprovided, the at least one polar control retarder may further comprise apassive retarder having an optical axis perpendicular to the plane ofthe retarder, the passive retarder having a retardance for light of awavelength of 550 nm in a range from −300 nm to −900 nm, preferably in arange from −450 nm to −800 nm and most preferably in a range from −500nm to −725 nm.

Alternatively, where two surface alignment layers providing homeotropicalignment are provided, the at least one polar control retarder furthercomprises a pair of passive retarders which have optical axes in theplane of the retarders that are crossed, each passive retarder of thepair of passive retarders having a retardance for light of a wavelengthof 550 nm in a range from 300 nm to 800 nm, preferably in a range from500 nm to 700 nm and most preferably in a range from 550 nm to 675 nm.Advantageously, in this case high transmission and low reflectivity maybe provided over a wide field of view with no voltage applied. Further anarrow field of view may be provided in a lateral direction in a privacymode of operation, with low power consumption.

Where the at least one polar control retarder comprises a switchable LCretarder, in another alternative the switchable LC retarder may comprisetwo surface alignment layers disposed adjacent to the layer of LCmaterial and on opposite sides thereof and each arranged to providehomogeneous alignment in the adjacent LC material. Advantageously incomparison to homeotropic alignment on opposite sides of the LC,increased resilience to the visibility of flow of LC material duringapplied pressure may be achieved.

The layer of LC material of the switchable LC retarder may comprise a LCmaterial with a positive dielectric anisotropy. The layer of LC materialmay have a retardance for light of a wavelength of 550 nm in a rangefrom 500 nm to 900 nm, preferably in a range from 600 nm to 850 nm andmost preferably in a range from 700 nm to 800 nm.

Where two surface alignment layers providing homogeneous alignment areprovided, the at least one polar control retarder may further comprise apassive retarder having an optical axis perpendicular to the plane ofthe retarder, the passive retarder having a retardance for light of awavelength of 550 nm in a range from −300 nm to −700 nm, preferably in arange from −350 nm to −600 nm and most preferably in a range from −400nm to −500 nm.

Alternatively, where the two surface alignment layers providinghomogeneous alignment are provided, the at least one polar controlretarder may further comprise a pair of passive retarders which haveoptical axes in the plane of the retarders that are crossed, eachpassive retarder of the pair of passive retarders having a retardancefor light of a wavelength of 550 nm in a range from 300 nm to 800 nm,preferably in a range from 350 nm to 650 nm and most preferably in arange from 450 nm to 550 nm.

The field of view using a pair of passive retarders which have opticalaxes in the plane of the retarders that are crossed may have improvedreduction of luminance and increase of reflectivity in privacy mode ofoperation.

Where the at least one polar control retarder comprises a switchable LCretarder, in another alternative the switchable LC retarder may comprisetwo surface alignment layers disposed adjacent to the layer of LCmaterial and on opposite sides thereof, one of the surface alignmentlayers being arranged to provide homeotropic alignment in the adjacentLC material and the other of the surface alignment layers being arrangedto provide homogeneous alignment in the adjacent LC material.

When the surface alignment layer arranged to provide homogeneousalignment is between the layer of LC material and the polar controlretarder, the layer of LC material may have a retardance for light of awavelength of 550 nm in a range from 700 nm to 2000 nm, preferably in arange from 1000 nm to 1500 nm and most preferably in a range from 1200nm to 1500 nm.

When the surface alignment layer arranged to provide homogeneousalignment is between the layer of LC material and the polar controlretarder, the at least one polar control retarder may further comprise apassive retarder having its optical axis perpendicular to the plane ofthe retarder, the at least one passive retarder having a retardance forlight of a wavelength of 550 nm in a range from −400 nm to −1800 nm,preferably in a range from −700 nm to −1500 nm and most preferably in arange from −900 nm to −1300 nm.

When the surface alignment layer arranged to provide homogeneousalignment is between the layer of LC material and the polar controlretarder, the at least one polar control retarder may further comprise apair of passive retarders which have optical axes in the plane of theretarders that are crossed, each retarder of the pair of retardershaving a retardance for light of a wavelength of 550 nm in a range from400 nm to 1800 nm, preferably in a range from 700 nm to 1500 nm and mostpreferably in a range from 900 nm to 1300 nm. Advantageously increasedresilience to the visibility of flow of LC material during appliedpressure may be achieved.

When the surface alignment layer arranged to provide homeotropicalignment is between the layer of LC material and the polar controlretarder, the layer of LC material may have a retardance for light of awavelength of 550 nm in a range from 500 nm to 1800 nm, preferably in arange from 700 nm to 1500 nm and most preferably in a range from 900 nmto 1350 nm.

When the surface alignment layer arranged to provide homeotropicalignment is between the layer of LC material and the polar controlretarder, the at least one polar control retarder may further comprise apassive retarder having its optical axis perpendicular to the plane ofthe retarder, the at least one passive retarder having a retardance forlight of a wavelength of 550 nm in a range from −300 nm to −1600 nm,preferably in a range from −500 nm to −1300 nm and most preferably in arange from −700 nm to −1150 nm.

When the surface alignment layer arranged to provide homeotropicalignment is between the layer of LC material and the polar controlretarder, the at least one polar control retarder may further comprise apair of passive retarders which have optical axes in the plane of theretarders that are crossed, each retarder of the pair of retardershaving a retardance for light of a wavelength of 550 nm in a range from400 nm to 1600 nm, preferably in a range from 600 nm to 1400 nm and mostpreferably in a range from 800 nm to 1300 nm. Advantageously incomparison to homeotropic alignment on opposite sides of the LC,increased resilience to the visibility of flow of LC material duringapplied pressure may be achieved.

Each alignment layer may have a pretilt having a pretilt direction witha component in the plane of the layer of LC material that is parallel oranti-parallel or orthogonal to the electric vector transmissiondirection of the reflective polariser. Advantageously high luminance maybe achieved for head-on viewing positions.

Each alignment layer may have a pretilt having a pretilt direction witha component in the plane of the layer of LC material that is parallel oranti-parallel or orthogonal to the electric vector transmissiondirection of the reflective polariser.

Where the at least one polar control retarder comprises a switchable LCretarder, the at least one passive retarder may further comprise twopassive retarders, the switchable LC retarder being provided between thetwo passive retarders. The display device may further comprise atransmissive electrode and LC surface alignment layer formed on a sideof each of the two passive retarders adjacent the switchable LCretarder. The display device may further comprise first and secondsubstrates between which the switchable LC retarder is provided, thefirst and second substrates each comprising one of the two passiveretarders. The two passive retarders may each comprise a passiveretarder having an optical axis perpendicular to the plane of theretarder with a total retardance for light of a wavelength of 550 nm ina range from −300 nm to −700 nm, preferably in a range from −350 nm to−600 nm and most preferably in a range from −400 nm to −500 nm. Each ofthe two passive retarders may have an optical axis in the plane of thepassive retarder, wherein the optical axes are crossed, and each passiveretarder of the pair of passive retarders having a retardance for lightof a wavelength of 550 nm in a range from 150 nm to 800 nm, preferablyin a range from 200 nm to 700 nm and most preferably in a range from 250nm to 600 nm. Advantageously thickness, cost and complexity may bereduced.

The switchable LC retarder may further comprise transmissive electrodesarranged to apply a voltage for controlling the layer of LC material.The transmissive electrodes may be on opposite sides of the layer of LCmaterial. The display device may further comprise a control systemarranged to control the voltage applied across the electrodes of theswitchable LC retarder. Advantageously the display may be controlled toswitch between privacy and public modes of operation.

The electrodes may be patterned to provide at least two pattern regions.Advantageously a camouflage pattern may be applied in privacy mode forluminance and reflectivity, and head-on luminance and reflectivity maybe substantially unmodified.

The at least one polar control retarder may comprise at least onepassive retarder which is arranged to introduce no net relative phaseshift to orthogonal polarisation components of light passed by thereflective polariser along an axis along a normal to the plane of the atleast one passive retarder and to introduce a net relative phase shiftto orthogonal polarisation components of light passed by the reflectivepolariser along an axis inclined to a normal to the plane of the atleast one passive retarder. Advantageously thickness and cost may bereduced and efficiency may be increased if no switchable LC polarcontrol retarder is provided.

The at least one polar control retarder may comprise at least onepassive retarder. The at least one passive retarder may comprise atleast two passive retarders with at least two different orientations ofoptical axes. Advantageously a low cost privacy display and low straylight display may be provided.

In one alternative, the at least one passive retarder may comprise aretarder having an optical axis perpendicular to the plane of theretarder. Advantageously thickness may be reduced.

In another alternative, the at least one passive retarder may comprise apair of passive retarders which have optical axes in the plane of theretarders that are crossed. Advantageously the cost of the passiveretarder may be reduced and high uniformity stretched films used for thepassive retarder.

The pair of retarders may have optical axes that extend at 45° and at135°, respectively, with respect to an electric vector transmissiondirection of the output polariser.

The display device may further comprise an additional pair of passiveretarders disposed between the first-mentioned pair of passive retardersand which have optical axes in the plane of the passive retarders thatare crossed. Advantageously a privacy display or low stray light displaymay be provided for both landscape and portrait orientations. In anautomotive vehicle, reflections from windscreens and other glasssurfaces can be reduced.

The additional pair of passive retarders may have optical axes that eachextend at 0° and at 90°, respectively, with respect to an electricvector transmission direction that is parallel to the electric vectortransmission of the output polariser. Advantageously high VSL may beprovided in polar regions with some rotational symmetry.

In another alternative, the at least one passive polar control retardermay comprise a retarder having an optical axis that is oriented with acomponent perpendicular to the plane of the retarder and a component inthe plane of the retarder. The component in the plane of the passiveretarder may extend at 0°, with respect to an electric vectortransmission direction that is parallel or perpendicular to the electricvector transmission of the display polariser. The at least one passivepolar control retarder may further comprise a passive retarder having anoptical axis perpendicular to the plane of the passive retarder or apair of passive retarders which have optical axes in the plane of thepassive retarders that are crossed.

Advantageously a privacy display may be provided that achieves reductionof luminance and increase of reflections in the lateral direction withlow cost and complexity. A mobile display may be rotated about ahorizontal axis while achieving comfortable image visibility for aprimary user.

The display device may further comprise at least one further polarcontrol retarder arranged between the output polariser and thereflective polariser. Advantageously further modification of thefield-of-view profile may be provided for transmitted light. Luminancemay be reduced to a snooper while the primary user may observe asubstantially the same luminance.

The display device may further comprise a backlight arranged to outputlight, wherein the SLM is a transmissive SLM arranged to receive outputlight from the backlight wherein the backlight provides a luminance atpolar angles to the normal to the SLM greater than 45 degrees that is atmost 30% of the luminance along the normal to the SLM, preferably atmost 20% of the luminance along the normal to the SLM, and mostpreferably at most 10% of the luminance along the normal to the SLM.Advantageously a high VSL may be provided with low thickness and lowcost. Further the VSL may be high in environments with reduced ambientilluminance.

A further additional polariser may be arranged between the further polarcontrol retarder and the reflective polariser. The display device mayfurther comprise at least one further polar control retarder and afurther additional polariser, wherein the at least one further polarcontrol retarder is arranged between the first-mentioned additionalpolariser and the further additional polariser. Advantageously luminancemay be reduced to a snooper.

The at least one further polar control retarder may comprise at leastone further passive retarder. Advantageously the increase in thicknessand cost may be small.

The first-mentioned at least one polar control retarder may comprise afirst switchable LC retarder comprising a first layer of LC material,and the at least one further polar control retarder may comprise asecond switchable LC retarder comprising a second layer of LC material.The further switchable LC retarder may comprise at least one surfacealignment layer disposed adjacent the LC material having a pretilthaving a pretilt direction with a component in the plane of the layer ofLC material that is aligned parallel or antiparallel or orthogonal tothe reflective polariser.

Advantageously the field of view in the public mode of operation may besubstantially unmodified while further modification of the field-of-viewprofile may be provided for transmitted light in the privacy mode ofoperation. Luminance may be reduced to a snooper while the primary usermay observe a substantially the same luminance. The first and second LCretarders may have retardances that are different. Chromatic variationswith viewing angle may be reduced.

The electric vector transmission direction of the reflective polarisermay be parallel to the electric vector transmission direction of theadditional polariser and/or parallel to the electric vector transmissiondirection of the output polariser.

The layers of LC material of each of the first and second switchable LCretarders may have a retardance for light of a wavelength of 550 nm in arange from 450 nm to 850 nm, preferably in a range from 500 nm to 750 nmand most preferably in a range from 550 nm to 650 nm. VSL at high polarviewing angles may be increased.

The first-mentioned at least one polar control retarder furthercomprises a pair of passive retarders which have optical axes in theplane of the retarders that are crossed, wherein the first of the pairof passive retarders has an optical axis that extends at 45° and 135°,respectively, with respect to an electric vector transmission directionof the output polariser, and the second of the pair of passive retardershas an optical axis that extends at 135° with respect to the electricvector transmission direction of the output polariser; and the at leastone further polar control retarder comprises a further pair of passiveretarders which have optical axes in the plane of the retarders that arecrossed, wherein the first of the further pair of passive retarders hasan optical axis that extends at 45° and 135°, respectively with respectto an electric vector transmission direction of the output polariser;and the optical axes of the one of the first-mentioned pair of passiveretarders and the one of the further pair of passive retarders that areclosest to each other extend in the same direction.

Advantageously the colour appearance of reflected and transmitted lightto an off-axis snooper may be symmetric for positive and negativelateral viewing angles. The minimum VSL may be increased.

Each passive retarder of the first-mentioned pair of passive retarders,and each passive retarder of the further pair of passive retarders, hasa retardance for light of a wavelength of 550 nm in a range from 300 nmto 800 nm, preferably in a range from 350 nm to 650 nm and mostpreferably in a range from 400 nm to 550 nm. VSL at high polar viewingangles may be increased.

The display device may further comprise: a backlight arranged to outputlight, wherein the SLM is a transmissive SLM arranged to receive outputlight from the backlight, and the SLM further comprises an inputpolariser arranged on the input side of the SLM, the input polariserbeing a linear polariser; and a further additional polariser arranged onthe input side of the input polariser, the further additional polariserbeing a linear polariser: and at least one further polar controlretarder arranged between the further additional polariser and the inputpolariser. Advantageously the thickness increase between the SLM andviewer is reduced. Increased image fidelity may be provided anddiffusion may be increased to reduce the appearance of specular frontsurface reflections to the head-on user. The number of lamination stepsmay be reduced, and VSL may be increased. A public mode may be providedwith wide viewing angle.

The display device may further comprise a control system arranged tocontrol apply a common voltage across the first and second switchable LCretarders, and wherein the LC material of the first LC retarder isdifferent from the LC material of the second LC retarder. Advantageouslythe cost of the control system may be reduced. Chromatic variations withviewing angle may be reduced.

The reflective polariser and the output polariser may have electricvector transmission directions that are parallel. The reflectivepolariser and the additional polariser may have electric vectortransmission directions that are parallel. The reflective polariser andthe additional polariser may have electric vector transmissiondirections that are not parallel, and the display device may furthercomprise a rotator retarder arranged between the reflective polariserand the additional polariser, the rotator retarder being arranged torotate a polarisation direction of polarised light incident thereonbetween the electric vector transmission directions of the displaypolariser and the additional polariser. Advantageously high efficiencymay be provided. The additional polariser may be aligned with anelectric vector transmission direction to transmit light throughpolarised sunglasses for typical user orientations. SLMs withnon-parallel output electric vector transmission directions such asTN-LCD may be used.

According to a second aspect of the present disclosure there is provideda view angle control optical element for application to the output sideof a display device for use in ambient illumination comprising a SLMarranged to output light: wherein the SLM comprises an output polariserarranged on the output side of the SLM; the view angle control opticalelement comprising an additional polariser; a reflective polariserarranged between the output polariser and the additional polariser onapplication of the view angle control optical element to the displaydevice: and at least one polar control retarder arranged between thereflective polariser and the additional polariser, wherein the at leastone polar control retarder is capable of simultaneously introducing nonet relative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser along an axis along a normal to theplane of the at least one polar control retarder and introducing arelative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser along an axis inclined to a normal tothe plane of the at least one polar control retarder.

Advantageously an after-market element may be attached to displays bydisplay users. The element does not require complex alignment. Moirébeating between the element and the pixels of the display is not presentand selection of the component with regards to pixel pitch is notrequired. Inventory cost is reduced. Alternatively, the view anglecontrol optical element may be conveniently factory fitted into displaymodules.

The various features and alternatives set out above with respect to thefirst aspect of the present disclosure may similarly be applied to thesecond aspect of the present disclosure.

According to a third aspect of the present disclosure there is provideda display device comprising: a SLM: a display polariser arranged on atleast one side of the SLM, the display polariser being a linearpolariser; and a first additional polariser arranged on the same side ofthe SLM as one of the at least one display polarisers, the firstadditional polariser being a linear polariser; and first plural polarcontrol retarders arranged between the first additional polariser andthe one of the at least one display polarisers: a further additionalpolariser arranged on the same side of the SLM as said one of the atleast one display polarisers, outside the first additional polariser,the further additional polariser being a linear polariser; and a furtherplural polar control retarders arranged between the further firstadditional polariser and the one of the at least one display polarisersfurther additional polariser; wherein the first-mentioned plural polarcontrol retarders comprise a pair of passive retarders which haveoptical axes in the plane of the retarders that are crossed, wherein thefirst of the pair of passive retarders has an optical axis that extendsat 45° with respect to an electric vector transmission direction of theoutput polariser, and the second of the pair of passive retarders has anoptical axis that extends at 135° with respect to the electric vectortransmission direction of the display polariser that is an outputpolariser and extend at 45° and 135°, respectively, with respect to anelectric vector transmission direction of the output polariser, andwherein the further plural polar control retarders comprise a furtherpair of passive retarders which have optical axes in the plane of theretarders that are crossed, wherein the first of the further pair ofpassive retarders has an optical axis that extends at 135° with respectto an electric vector transmission direction of the output polariser,and the second of the further pair of passive retarders has an opticalaxis that extends at 45° with respect to the electric vectortransmission direction of the display polariser that is the outputpolariser and extend at 45° and 135°, respectively with respect to anelectric vector transmission direction of the output polariser, and theoptical axes of the one of the first pair of passive polar controlretarders and the one of the further pair of passive polar controlretarders that are closest to each other extend in the same direction.

Advantageously a switchable privacy display may be provided with highimage visibility over a wide field of view in a public mode ofoperation. A wide angle backlight may be provided, with reduced cost andhigher ruggedness in comparison to collimated backlights. In a privacymode of operation, high VSLs may be achieved over a wide field of viewin which an off-axis snooper may be positioned, with low displayreflectivity. The retarders and additional polarisers may be arrangedbetween the backlight and the SLM so that diffusers with surfaceroughness may be arranged on the front surface of the display tominimise the visibility of frontal reflections while achieving highpixel fidelity. Chromaticity and luminance roll-offs may be symmetric.

According to a fourth aspect of the present disclosure there is provideda transmissive SLM arranged to receive output light from the backlight;an input polariser arranged on the input side of the SLM and an outputpolariser arranged on the output side of the SLM, the input polariserand the output polariser being linear polarisers; a first additionalpolariser arranged on the output side of output polariser, the firstadditional polariser being a linear polariser; and first polar controlretarders arranged between the first additional polariser and the outputpolariser; a further additional polariser arranged between the backlightand input polariser, the further additional polariser being a linearpolariser: and further polar control retarders arranged between thefirst additional polariser and the input polariser; wherein the firstpolar control retarders comprise a pair of passive retarders which haveoptical axes in the plane of the retarders that are crossed and extendat 45° and 135°, respectively, with respect to an electric vectortransmission direction of the output polariser, the further polarcontrol retarders comprise a further pair of passive retarders whichhave optical axes in the plane of the retarders that are crossed andextend at 45° and 135°, respectively with respect to an electric vectortransmission direction of the output polariser, and the optical axes ofthe one of the first pair of passive polar control retarders and the oneof the further pair of passive polar control retarders that are closestto each other extend in the same direction.

Advantageously a switchable privacy display may be provided with highimage visibility over a wide field of view in a public mode ofoperation. A wide angle backlight may be provided, with reduced cost andhigher ruggedness in comparison to collimated backlights. In a privacymode of operation, high VSLs may be achieved over a wide field of viewin which an off-axis snooper may be positioned, with low displayreflectivity. Some of the retarders and additional polarisers may bearranged between the backlight and the SLM so that diffusers withsurface roughness may be arranged on the front surface of the display tominimise the visibility of frontal reflections while achieving highpixel fidelity and high image contrast. Chromaticity and luminanceroll-offs may be symmetric. Scatter from the SLM may not impact thelight that transmits through one of the retarders and the additionalpolariser so that VSL may be increased.

Embodiments of the present disclosure may be used in a variety ofoptical systems. The embodiments may include or work with a variety ofprojectors, projection systems, optical components, displays,microdisplays, computer systems, processors, self-contained projectorsystems, visual and/or audio-visual systems and electrical and/oroptical devices. Aspects of the present disclosure may be used withpractically any apparatus related to optical and electrical devices,optical systems, presentation systems or any apparatus that may containany type of optical system. Accordingly, embodiments of the presentdisclosure may be employed in optical systems, devices used in visualand/or optical presentations, visual peripherals and so on and in anumber of computing environments.

Before proceeding to the disclosed embodiments in detail, it should beunderstood that the disclosure is not limited in its application orcreation to the details of the particular arrangements shown, becausethe disclosure is capable of other embodiments. Moreover, aspects of thedisclosure may be set forth in different combinations and arrangementsto define embodiments unique in their own right. Also, the terminologyused herein is for the purpose of description and not of limitation.

These and other advantages and features of the present disclosure willbecome apparent to those of ordinary skill in the art upon reading thisdisclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingFIGURES, in which like reference numbers indicate similar parts, and inwhich:

FIG. 1A is a diagram illustrating in side perspective view a switchableprivacy display for use in ambient illumination comprising atransmissive SLM, reflective polariser and compensated switchableretarder:

FIG. 1B is a diagram illustrating in side perspective view a switchableprivacy display for use in ambient illumination comprising an emissiveSLM and compensated switchable retarder:

FIG. 2A is a diagram illustrating in front view alignment of opticallayers in the optical stack of FIG. 1A;

FIG. 2B is a diagram illustrating in side perspective view a view anglecontrol element comprising a reflective polariser, a passive polarcontrol retarder, a switchable LC retarder and an additional polariser:

FIG. 3 is a diagram illustrating in perspective side view an arrangementof a switchable LC retarder comprising a passive negative C-plate polarcontrol retarder in a privacy mode:

FIG. 4A is a diagram illustrating in side view propagation of outputlight from a SLM through the optical stack of FIG. 1A in a privacy mode:

FIG. 4B is a graph illustrating the variation of output luminance withpolar direction for the transmitted light rays in FIG. 4A;

FIG. 5A is a diagram illustrating in top view propagation of ambientillumination light through the optical stack of FIG. 1A in a privacymode;

FIG. 5B is a graph illustrating the variation of reflectivity with polardirection for the reflected light rays in FIG. 5A;

FIG. 5C is a graph illustrating a measurement of the variation ofreflectivity with lateral direction for the reflected light rays in FIG.5A:

FIG. 6A is a diagram illustrating in front perspective view observationof transmitted output light for a display in privacy mode;

FIG. 6B is a diagram illustrating in front perspective view observationof reflected ambient light from interface surfaces of a display;

FIG. 6C is a diagram illustrating in front perspective view observationof reflected ambient light for the display of FIG. 1A and FIG. 1B inprivacy mode;

FIG. 7A is a diagram illustrating in front perspective views theappearance of the display of FIG. 1A and FIG. 1B in privacy mode:

FIG. 7B is a graph illustrating the variation of perceived dynamic rangeagainst ambient illuminance for an off-axis snooper of the switchableprivacy display of FIG. 1A and FIG. 1B in a privacy mode forarrangements with and without the reflective polariser;

FIG. 7C is a graph illustrating the variation of VSL with polardirection for a display of FIG. 1A comprising a collimated backlight;

FIG. 7D is a graph illustrating the variation of VSL with polardirection for a display comprising no plural retarders:

FIG. 8A is a diagram illustrating in side view an automotive vehiclewith a switchable directional display arranged within the vehicle cabinfor both entertainment and sharing modes;

FIG. 8B is a diagram illustrating in top view an automotive vehicle witha switchable directional display arranged within the vehicle cabin in anentertainment mode;

FIG. 8C is a diagram illustrating in top view an automotive vehicle witha switchable directional display arranged within the vehicle cabin in asharing mode;

FIG. 8D is a diagram illustrating in top view an automotive vehicle witha switchable directional display arranged within the vehicle cabin forboth night-time and day-time modes;

FIG. 8E is a diagram illustrating in side view an automotive vehiclewith a switchable directional display arranged within the vehicle cabinin a night-time mode;

FIG. 8F is a diagram illustrating in side view an automotive vehiclewith a switchable directional display arranged within the vehicle cabinin a day-time mode;

FIG. 9A is a diagram illustrating in perspective side view anarrangement of a switchable retarder in a public mode wherein theswitchable retarder comprises a switchable LC layer with homeotropicalignment and a passive C-plate polar control retarder;

FIG. 9B is a diagram illustrating in side view propagation of outputlight from a SLM through the optical stack of FIG. 1A in a public mode;

FIG. 9C is a graph illustrating the variation of output luminance withpolar direction for the transmitted light rays in FIG. 9B;

FIG. 9D is a diagram illustrating in top view propagation of ambientillumination light through the optical stack of FIG. 1A in a publicmode;

FIG. 9E is a graph illustrating the variation of reflectivity with polardirection for the reflected light rays in FIG. 9D;

FIG. 10A is a diagram illustrating in front perspective view observationof transmitted output light for a display in public mode;

FIG. 10B is a diagram illustrating in front perspective view observationof reflected ambient light from the switchable display of FIG. 1A inpublic mode;

FIG. 10C is a diagram illustrating in front perspective views theappearance of the display of FIG. 1A in public mode;

FIG. 11A is a diagram illustrating in perspective side view anarrangement of a switchable retarder in a public mode wherein theswitchable retarder comprises a switchable LC layer with homogeneousalignment and crossed A-plate polar control retarders;

FIG. 11B is a graph illustrating the variation of output luminance withpolar direction for transmitted light rays in FIG. 11A in a privacymode:

FIG. 11C is a graph illustrating the variation in reflectivity withpolar direction for reflected light rays in FIG. 11A in a privacy mode:

FIG. 11D is a graph illustrating the variation of output luminance withpolar direction for transmitted light rays in FIG. 11A in a public mode;

FIG. 11E is a graph illustrating the variation in reflectivity withpolar direction for reflected light rays in FIG. 11A in a public mode;

FIG. 11F is a diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy modecomprising the crossed A-plate passive polar control retarders andhomogeneously aligned switchable LC retarder, further comprising apassive rotation retarder;

FIG. 12A is a diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode comprising ahomogeneously aligned switchable LC retarder and a passive negativeC-plate retarder driven with a first voltage:

FIG. 12B is a diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode comprising ahomogeneously aligned switchable LC retarder and a passive negativeC-plate retarder driven with a second voltage different to the firstvoltage:

FIG. 12C is a graph illustrating the variation of output luminance withpolar direction for transmitted light rays in FIG. 12A in a privacymode;

FIG. 12D is a graph illustrating the variation in reflectivity withpolar direction for reflected light rays in FIG. 12A in a privacy mode;

FIG. 12E is a graph illustrating the variation of output luminance withpolar direction for transmitted light rays in FIG. 12B in a public mode;

FIG. 12F is a graph illustrating the variation in reflectivity withpolar direction for reflected light rays in FIG. 12B in a public mode;

FIG. 13A is a diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode comprising ahomogeneously aligned switchable LC retarder;

FIG. 13B is a graph illustrating the variation of output luminance withpolar direction for transmitted light rays in FIG. 13A in a privacymode;

FIG. 13C is a graph illustrating the variation in reflectivity withpolar direction for reflected light rays in FIG. 13A in a privacy mode;

FIG. 13D is a graph illustrating the variation of output luminance withpolar direction for transmitted light rays in FIG. 13A in a public mode:

FIG. 13E is a graph illustrating the variation in reflectivity withpolar direction for reflected light rays in FIG. 13A in a public mode:

FIG. 13F is a diagram illustrating in side perspective view a view anglecontrol element comprising a reflective polariser, a switchable LCretarder and an additional polariser;

FIG. 14A is a diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode comprisingcrossed A-plate passive retarders and homeotropically aligned switchableLC retarder;

FIG. 14B is a graph illustrating the variation of output luminance withpolar direction for transmitted light rays in FIG. 14A in a privacymode;

FIG. 14C is a graph illustrating the variation in reflectivity withpolar direction for reflected light rays in FIG. 14A in a privacy mode;

FIG. 14D is a diagram illustrating in perspective side view anarrangement of a switchable retarder in a public mode comprising crossedA-plate passive retarders and homeotropically aligned switchable LCretarder:

FIG. 14E is a graph illustrating the variation of output luminance withpolar direction for transmitted light rays in FIG. 14D in a public mode;

FIG. 14F is a graph illustrating the variation in reflectivity withpolar direction for reflected light rays in FIG. 14D in a privacy mode;

FIG. 15A is a diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode comprising ahomogeneously and homeotropically aligned switchable LC retarder and apassive negative C-plate retarder:

FIG. 15B is a graph illustrating the variation of output luminance withpolar direction for transmitted light rays in FIG. 15A in a privacymode;

FIG. 15C is a graph illustrating the variation in reflectivity withpolar direction for reflected light rays in FIG. 15A in a privacy mode:

FIG. 15D is a graph illustrating the variation of output luminance withpolar direction for transmitted light rays in FIG. 15A in a public mode;

FIG. 15E is a graph illustrating the variation in reflectivity withpolar direction for reflected light rays in FIG. 15A in a public mode;

FIG. 16 is a diagram illustrating in side perspective view a switchableprivacy display for use in ambient illumination comprising anon-collimating backlight, a passive retarder arranged between areflective recirculation polariser and a transmissive SLM, a reflectivepolariser, a compensated switchable retarder and additional polariser;

FIG. 17A is a diagram illustrating in side perspective view a switchableprivacy display for use in ambient illumination comprising an emissiveSLM, a passive control retarder, a further additional polariser, areflective polariser, a compensated switchable retarder and anadditional polariser;

FIG. 17B is a diagram illustrating in side perspective view a view anglecontrol element comprising a passive control retarder, a firstadditional polariser, a reflective polariser, a passive polar controlretarder, a switchable LC retarder and a second additional polariser:

FIG. 18A is a diagram illustrating in side perspective view a switchableprivacy display for use in ambient illumination comprising a wide anglebacklight wherein first plural retarders are arranged between backlightand the SLM and further plural retarders are arranged to receive lightfrom the SLM;

FIG. 18B is a diagram illustrating in front view alignment of opticallayers of an optical stack comprising plural retarders arranged betweena reflective polariser and an additional polariser and further pluralretarders arranged between the input polariser and a further additionalpolariser of a transmissive SLM wherein plural retarders and furtherplural retarders each comprise crossed A-plates:

FIG. 18C is a graph illustrating the variation of logarithmic outputluminance with polar direction for transmitted light rays of pluralretarders comprising crossed passive A-plates and a homogeneouslyaligned switchable LC retarder;

FIG. 18D is a graph illustrating in a lateral direction the variation oflogarithmic output luminance with lateral viewing angle for transmittedlight rays of plural retarders comprising crossed passive A-plates and ahomogeneously aligned switchable LC retarder:

FIG. 18E is a diagram illustrating in side perspective view a switchableprivacy display for use in ambient illumination comprising an emissiveSLM, a first compensated switchable LC retarder, a first additionalpolariser, a reflective polariser, a second compensated switchable LCretarder and a second additional polariser;

FIG. 18F is a diagram illustrating in front view alignment of opticallayers of an optical stack comprising plural retarders arranged betweena reflective polariser and an additional polariser and further pluralretarders arranged between the output polariser and a further additionalpolariser which is the reflective polariser wherein the plural retardersand further plural retarders each comprise crossed A-plates;

FIG. 18G is a diagram illustrating in front view alignment of opticallayers of an optical stack comprising plural retarders arranged betweena further additional light absorbing polariser and an additionalpolariser and further plural retarders arranged between the outputpolariser and the further additional polariser wherein plural retardersand further plural retarders each comprise crossed A-plates;

FIG. 18H is a diagram illustrating in front view alignment of opticallayers of an optical stack for a transmissive SLM comprising pluralretarders arranged between a further additional light absorbingpolariser and an additional polariser and further plural retardersarranged between the input polariser and the further additionalpolariser wherein the plural retarders and further plural retarders eachcomprise crossed A-plates;

FIG. 18I is a diagram illustrating in front view alignment of opticallayers of an optical stack for a transmissive SLM comprising pluralretarders arranged between a further additional polariser and the inputpolariser of a transmissive SLM and plural retarders arranged betweenthe output polariser and an additional polariser wherein the pluralretarders and further plural retarders each comprise crossed A-plates:

FIG. 18J is a diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode comprising afirst negative C-plate passive retarder and first homogeneously alignedswitchable LC retarder arranged between the output polariser and areflective polariser; and a second negative C-plate passive retarder andsecond homogeneously aligned switchable LC retarder arranged between thereflective polariser and a further additional polariser;

FIG. 18K is a diagram illustrating in side perspective view a view anglecontrol element comprising a first compensated switchable LC retarder, afirst additional polariser, a reflective polariser, a second compensatedswitchable LC retarder, and a second additional polariser;

FIG. 19A is a diagram illustrating in top view an automotive vehiclewith a switchable directional display arranged within the vehicle cabinfor day-time and/or sharing modes;

FIG. 19B is a diagram illustrating in side view an automotive vehiclewith a switchable directional display arranged within the vehicle cabinfor day-time and/or sharing modes:

FIG. 19C is a diagram illustrating in top view an automotive vehiclewith a switchable directional display arranged within the vehicle cabinfor night-time and/or entertainment modes;

FIG. 19D is a diagram illustrating in side view an automotive vehiclewith a switchable directional display arranged within the vehicle cabinfor night-time and/or entertainment modes;

FIG. 20A is a diagram illustrating in side perspective view a privacydisplay for use in ambient illumination comprising a backlight, atransmissive SLM, a reflective polariser, a retarder stack and anadditional polariser;

FIG. 20B is a diagram illustrating in side perspective view a view anglecontrol element comprising a reflective polariser, a retarder stack andan additional polariser;

FIG. 20C is a diagram illustrating in side perspective view a view anglecontrol element comprising a first retarder stack and an additionalpolariser; a reflective polariser; a second retarder stack and a furtheradditional polariser;

FIG. 20D is a diagram illustrating in side perspective view a privacydisplay for use in ambient illumination comprising a backlight, areflective recirculation polariser, an input retarder stack, atransmissive SLM, a reflective polariser, a retarder stack and anadditional polariser;

FIG. 21A is a diagram illustrating in side perspective view an opticalstack of a passive retarder comprising a negative C-plate and arrangedto provide field-of-view modification of a display device;

FIG. 21B is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in the passive retarderof FIG. 21A;

FIG. 21C is a diagram illustrating in side perspective view an opticalstack of a passive retarder comprising a negative O-plate tilted in aplane orthogonal to the display polariser electric vector transmissiondirection and a negative C-plate and arranged to provide field-of-viewmodification of a display device;

FIG. 21D is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in the passive retarderof FIG. 21C;

FIG. 21E is a diagram illustrating in side perspective view an opticalstack of a passive retarder comprising a positive O-plate tilted in aplane orthogonal to the display polariser electric vector transmissiondirection and crossed A-plates and arranged to provide field-of-viewmodification of a display device;

FIG. 21F is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in the passive retarderof FIG. 21E;

FIG. 22A is a diagram illustrating in side perspective view an opticalstack arranged to provide field-of-view modification of a display devicecomprising two pairs of crossed A-plates;

FIG. 22B is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in the passive retarderof FIG. 22A;

FIG. 23A and FIG. 23B are diagrams illustrating in side view a privacydisplay for use in ambient illumination comprising a transmissive SLM, areflective polariser, a LC retarder, compensating retarders and anadditional polariser;

FIG. 24A is a diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy modecomprising a homogeneously aligned switchable LC retarder arrangedbetween first and second C-plate passive polar control retarders:

FIG. 24B and FIG. 24C are graphs illustrating the variation of outputtransmission with polar direction for transmitted light rays in theoptical stack of FIG. 24A in a public mode and a privacy moderespectively;

FIG. 24D is a graph illustrating the variation in reflectivity withpolar direction for reflected light rays in FIG. 24A in a privacy mode;

FIG. 25A is a diagram illustrating in perspective side view a displaycomprising a switchable compensated retarder arranged between first andsecond C-plate passive polar control retarder substrates;

FIG. 25B is a diagram illustrating in side view part of a displaycomprising a switchable compensated retarder arranged between first andsecond C-plate passive polar control retarder substrates;

FIG. 25C is a diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a public modecomprising a homogeneously aligned switchable LC retarder arrangedbetween first and second crossed A-plate passive polar controlretarders;

FIG. 25D and FIG. 25E are graphs illustrating the variation of outputtransmission with polar direction for transmitted light rays for thearrangement of FIG. 25C in wide angle and privacy modes respectively;

FIG. 26A is a diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode comprising anegative C-plate passive retarder and homeotropically aligned switchableLC retarder further comprising a patterned electrode layer:

FIG. 26B is a diagram illustrating in perspective front viewillumination of a primary viewer and a snooper by a camouflagedluminance controlled privacy display:

FIG. 26C is a diagram illustrating in perspective side view illuminationof a snooper by a camouflaged luminance controlled privacy display;

FIG. 27A is a diagram illustrating in perspective side view anarrangement of a homogeneously aligned switchable LC retarder;

FIG. 27B is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in FIG. 27A for a firstapplied voltage;

FIG. 27C is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in FIG. 27A for a secondapplied voltage greater than the first applied voltage;

FIG. 27D is a diagram illustrating in perspective side view a C-platearranged between parallel polarisers;

FIG. 27E is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in FIG. 27D;

FIG. 28A is a diagram illustrating in perspective side view anarrangement of a homogeneously aligned switchable LC retarder arrangedbetween parallel polarisers in series with a C-plate arranged betweenparallel polarisers;

FIG. 28B is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in FIG. 28A for a firstapplied voltage;

FIG. 28C is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in FIG. 28A for a secondapplied voltage greater than the first applied voltage;

FIG. 29A is a diagram illustrating in perspective side view anarrangement of a homogeneously aligned switchable LC retarder in serieswith a C-plate polar control retarder wherein the homogeneously alignedswitchable LC and C-plate polar control retarder are arranged between asingle pair of parallel polarisers;

FIG. 29B is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in FIG. 29A for a firstapplied voltage;

FIG. 29C is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in FIG. 29A for a secondapplied voltage greater than the first applied voltage:

FIG. 30A is a diagram illustrating in front perspective view adirectional backlight:

FIG. 30B is a diagram illustrating in front perspective view anon-directional backlight;

FIG. 30C is a graph illustrating variation with luminance with lateralviewing angle of displays with different fields of view;

FIG. 31A is a diagram illustrating in side view a switchable directionaldisplay apparatus comprising an imaging waveguide and switchable LCretarder;

FIG. 31B is a diagram illustrating in rear perspective view operation ofan imaging waveguide in a narrow angle mode;

FIG. 31C is a graph illustrating a field-of-view luminance plot of theoutput of FIG. 31B when used in a display apparatus with no switchableLC retarder;

FIG. 32A is a diagram illustrating in side view a switchable directionaldisplay apparatus comprising a switchable collimating waveguide and aswitchable LC retarder in a privacy mode;

FIG. 32B is a diagram illustrating in top view output of a collimatingwaveguide:

FIG. 32C is a graph illustrating an iso-luminance field-of-view polarplot for the display apparatus of FIG. 32A;

FIG. 33A is a diagram illustrating in perspective view illumination of aretarder layer by off-axis light:

FIG. 33B is a diagram illustrating in perspective view illumination of aretarder layer by off-axis light of a first linear polarization state at0 degrees:

FIG. 33C is a diagram illustrating in perspective view illumination of aretarder layer by off-axis light of a first linear polarization state at90 degrees;

FIG. 33D is a diagram illustrating in perspective view illumination of aretarder layer by off-axis light of a first linear polarization state at45 degrees:

FIG. 34A is a diagram illustrating in perspective view illumination of aC-plate retarder by off-axis polarised light with a positive elevation:

FIG. 34B is a diagram illustrating in perspective view illumination of aC-plate retarder by off-axis polarised light with a negative lateralangle;

FIG. 34C is a diagram illustrating in perspective view illumination of aC-plate retarder by off-axis polarised light with a positive elevationand negative lateral angle;

FIG. 34D is a diagram illustrating in perspective view illumination of aC-plate retarder by off-axis polarised light with a positive elevationand positive lateral angle;

FIG. 34E is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in FIGS. 34A-D;

FIG. 35A is a diagram illustrating in perspective view illumination ofcrossed A-plate retarder layers by off-axis polarised light with apositive elevation:

FIG. 35B is a diagram illustrating in perspective view illumination ofcrossed A-plate retarder layers by off-axis polarised light with anegative lateral angle;

FIG. 35C is a diagram illustrating in perspective view illumination ofcrossed A-plate retarder layers by off-axis polarised light with apositive elevation and negative lateral angle;

FIG. 35D is a diagram illustrating in perspective view illumination ofcrossed A-plate retarder layers by off-axis polarised light with apositive elevation and positive lateral angle; and

FIG. 35E is a graph illustrating the variation of output transmissionwith polar direction for transmitted light rays in FIGS. 35A-D.

DETAILED DESCRIPTION

Terms related to optical retarders for the purposes of the presentdisclosure will now be described.

In a layer comprising a uniaxial birefringent material there is adirection governing the optical anisotropy whereas all directionsperpendicular to it (or at a given angle to it) have equivalentbirefringence.

The optical axis of an optical retarder refers to the direction ofpropagation of a light ray in the uniaxial birefringent material inwhich no birefringence is experienced. This is different from theoptical axis of an optical system which may for example be parallel to aline of symmetry or normal to a display surface along which a principalray propagates.

For light propagating in a direction orthogonal to the optical axis, theoptical axis is the slow axis when linearly polarized light with anelectric vector direction parallel to the slow axis travels at theslowest speed. The slow axis direction is the direction with the highestrefractive index at the design wavelength. Similarly the fast axisdirection is the direction with the lowest refractive index at thedesign wavelength.

For positive dielectric anisotropy uniaxial birefringent materials theslow axis direction is the extraordinary axis of the birefringentmaterial. For negative dielectric anisotropy uniaxial birefringentmaterials the fast axis direction is the extraordinary axis of thebirefringent material.

The terms half a wavelength and quarter a wavelength refer to theoperation of a retarder for a design wavelength λ₀ that may typically bebetween 500 nm and 570 nm. In the present illustrative embodimentsexemplary retardance values are provided for a wavelength of 550 nmunless otherwise specified.

The retarder provides a relative phase shift between two orthogonalpolarization components of the light wave incident thereon and ischaracterized by the amount of relative phase. F, that it imparts on thetwo polarization components. In some contexts, the term “phase shift” isused without the word “relative” but still meaning relative phase shift.The relative phase shift is related to the birefringence Δn and thethickness d of the retarder by:

Γ=2·π·Δn·d/λ ₀  eqn. 1

In eqn. 1, Δn is defined as the difference between the extraordinary andthe ordinary index of refraction, i.e.

Δn=n _(e) −n _(o)  eqn. 2

For a half-wave retarder, the relationship between d, Δn, and λ₀ ischosen so that the phase shift between polarization components is r=n.For a quarter-wave retarder, the relationship between d, Δn, and λ₀ ischosen so that the phase shift between polarization components is Γ=π/2.

The term half-wave retarder herein typically refers to light propagatingnormal to the retarder and normal to the spatial light modulator (SLM).

Some aspects of the propagation of light rays through a transparentretarder between a pair of polarisers will now be described.

The state of polarisation (SOP) of a light ray is described by therelative amplitude and phase shift between any two orthogonalpolarization components. Transparent retarders do not alter the relativeamplitudes of these orthogonal polarisation components but act only ontheir relative phase. Providing a net phase shift between the orthogonalpolarisation components alters the SOP whereas maintaining net relativephase preserves the SOP.

A linear SOP has a polarisation component with a non-zero amplitude andan orthogonal polarisation component which has zero amplitude.

A linear polariser transmits a unique linear SOP that has a linearpolarisation component parallel to the electric vector transmissiondirection of the linear polariser and attenuates light with a differentSOP.

Absorbing polarisers are polarisers that absorb one polarisationcomponent of incident light and transmit a second orthogonalpolarisation component. Examples of absorbing linear polarisers aredichroic polarisers.

Reflective polarisers are polarisers that reflect one polarisationcomponent of incident light and transmit a second orthogonalpolarisation component. Examples of reflective polarisers that arelinear polarisers are multilayer polymeric film stacks such as DBEF™ orAPF™ from 3M Corporation, or wire grid polarisers such as ProFlux™ fromMoxtek. Reflective linear polarisers may further comprise cholestericreflective materials and a quarter waveplate arranged in series.

A retarder arranged between a linear polariser and a parallel linearanalysing polariser that introduces no relative net phase shift providesfull transmission of the light other than residual absorption within thelinear polariser.

A retarder that provides a relative net phase shift between orthogonalpolarisation components changes the SOP and provides attenuation at theanalysing polariser.

In the present disclosure an ‘A-plate’ refers to an optical retarderutilizing a layer of birefringent material with its optical axisparallel to the plane of the layer.

A ‘positive A-plate’ refers to positively birefringent A-plates, i.e.A-plates with a positive Δn.

In the present disclosure a ‘C-plate’ refers to an optical retarderutilizing a layer of birefringent material with its optical axisperpendicular to the plane of the layer. A ‘positive C-plate’ refers topositively birefringent C-plate, i.e. a C-plate with a positive Δn. A‘negative C-plate’ refers to a negatively birefringent C-plate, i.e. aC-plate with a negative Δn.

‘O-plate’ refers to an optical retarder utilizing a layer ofbirefringent material with its optical axis having a component parallelto the plane of the layer and a component perpendicular to the plane ofthe layer. A ‘positive O-plate’ refers to positively birefringentO-plates, i.e. O-plates with a positive Δn.

Achromatic retarders may be provided wherein the material of theretarder is provided with a retardance Δn·d that varies with wavelengthλ as

Δn·d/λ=κ  eqn. 3

where κ is substantially a constant.

Examples of suitable materials include modified polycarbonates fromTeijin Films. Achromatic retarders may be provided in the presentembodiments to advantageously minimise color changes between polarangular viewing directions which have low luminance reduction and polarangular viewing directions which have increased luminance reductions aswill be described below.

Various other terms used in the present disclosure related to retardersand to liquid crystals will now be described.

A liquid crystal cell has a retardance given by Δn·d where Δn is thebirefringence of the liquid crystal material in the liquid crystal celland d is the thickness of the liquid crystal cell, independent of thealignment of the liquid crystal material in the liquid crystal cell.

Homogeneous alignment refers to the alignment of liquid crystals inswitchable LCDs where molecules align substantially parallel to asubstrate. Homogeneous alignment is sometimes referred to as planaralignment. Homogeneous alignment may typically be provided with a smallpre-tilt such as 2 degrees, so that the molecules at the surfaces of thealignment layers of the liquid crystal cell are slightly inclined aswill be described below. Pretilt is arranged to minimise degeneracies inswitching of cells.

In the present disclosure, homeotropic alignment is the state in whichrod-like liquid crystalline molecules align substantiallyperpendicularly to the substrate. In discotic liquid crystalshomeotropic alignment is defined as the state in which an axis of thecolumn structure, which is formed by disc-like liquid crystallinemolecules, aligns perpendicularly to a surface. In homeotropicalignment, pretilt is the tilt angle of the molecules that are close tothe alignment layer and is typically close to 90 degrees and for examplemay be 88 degrees.

In a twisted liquid crystal layer a twisted configuration (also known asa helical structure or helix) of nematic liquid crystal molecules isprovided. The twist may be achieved by means of a non-parallel alignmentof alignment layers. Further, cholesteric dopants may be added to theliquid crystal material to break degeneracy of the twist direction(clockwise or anti-clockwise) and to further control the pitch of thetwist in the relaxed (typically undriven) state. A supertwisted liquidcrystal layer has a twist of greater than 180 degrees. A twisted nematiclayer used in SLMs typically has a twist of 90 degrees.

Liquid crystal molecules with positive dielectric anisotropy areswitched from a homogeneous alignment (such as an A-plate retarderorientation) to a homeotropic alignment (such as a C-plate or O-plateretarder orientation) by means of an applied electric field.

Liquid crystal molecules with negative dielectric anisotropy areswitched from a homeotropic alignment (such as a C-plate or O-plateretarder orientation) to a homogeneous alignment (such as an A-plateretarder orientation) by means of an applied electric field.

Rod-like molecules have a positive birefringence so that n_(e)>n_(o) asdescribed in equation 2. Discotic molecules have negative birefringenceso that n_(e)<n_(o).

Positive retarders such as A-plates, positive O-plates and positiveC-plates may typically be provided by stretched films or rod-like liquidcrystal molecules. Negative retarders such as negative C-plates may beprovided by stretched films or discotic like liquid crystal molecules.

Parallel liquid crystal cell alignment refers to the alignment directionof homogeneous alignment layers being parallel or more typicallyantiparallel. In the case of pre-tilted homeotropic alignment, thealignment layers may have components that are substantially parallel orantiparallel. Hybrid aligned liquid crystal cells may have onehomogeneous alignment layer and one homeotropic alignment layer. Twistedliquid crystal cells may be provided by alignment layers that do nothave parallel alignment, for example oriented at 90 degrees to eachother.

Transmissive SLMs may further comprise retarders between the inputdisplay polariser and the output display polariser for example asdisclosed in U.S. Pat. No. 8,237,876, which is herein incorporated byreference in its entirety. Such retarders (not shown) are in a differentplace to the passive retarders of the present embodiments. Suchretarders compensate for contrast degradations for off-axis viewinglocations, which is a different effect to the luminance reduction foroff-axis viewing positions of the present embodiments.

A private mode of operation of a display is one in which an observersees a low contrast sensitivity such that an image is not clearlyvisible. Contrast sensitivity is a measure of the ability to discernbetween luminances of different levels in a static image. Inversecontrast sensitivity may be used as a measure of visual security, inthat a high visual security level (VSL) corresponds to low imagevisibility.

For a privacy display providing an image to an observer, visual securitymay be given as:

VSL=(Y+R)/(Y−K)  eqn. 4

where VSL is the visual security level, Y is the luminance of the whitestate of the display at a snooper viewing angle, K is the luminance ofthe black state of the display at the snooper viewing angle and R is theluminance of reflected light from the display.

Panel contrast ratio is given as:

C=Y/K  eqn. 5

For high contrast optical LCD modes, the white state transmissionremains substantially constant with viewing angle. In the contrastreducing liquid crystal modes of the present embodiments, white statetransmission typically reduces as black state transmission increasessuch that

Y+K˜P·L  eqn. 6

The visual security level may then be further given as:

$\begin{matrix}{{VSL} = \frac{\left( {C + {{I \cdot \rho}\text{/}{\pi \cdot \left( {C + 1} \right)}\text{/}\left( {P \cdot L} \right)}} \right)}{\left( {C - 1} \right)}} & {{eqn}.\mspace{14mu} 7}\end{matrix}$

where off-axis relative luminance, P is typically defined as thepercentage of head-on luminance, L at the snooper angle and the displaymay have image contrast ratio C and the surface reflectivity is ρ.

The off-axis relative luminance, P is sometimes referred to as theprivacy level. However, such privacy level P describes relativeluminance of a display at a given polar angle compared to head-onluminance, and is not a measure of privacy appearance.

The display may be illuminated by Lambertian ambient illuminance I. Thusin a perfectly dark environment, a high contrast display has VSL ofapproximately 1.0. As ambient illuminance increases, the perceived imagecontrast degrades. VSL increases and a private image is perceived.

For typical liquid crystal displays the panel contrast C is above 100:1for almost all viewing angles, allowing the visual security level to beapproximated to:

VSL=1+I·ρ/(π·P·L)  eqn. 8

In comparison to privacy displays, desirably wide angle displays areeasily observed in standard ambient illuminance conditions. One measureof image visibility is given by the contrast sensitivity such as theMichelson contrast which is given by:

M=(I _(max) −I _(min))/(I _(max) +I _(min))  eqn. 9

and so:

M=((Y+R)−(K+R))/((Y+R)+(K+R))=(Y−K)/(Y+K+2·R)  eqn. 10

Thus the visual security level (VSL), is equivalent (but not identicalto) 1/M. In the present discussion, for a given off-axis relativeluminance, P the wide angle image visibility, W is approximated as

W=1/VSL=1/(1+I·ρ/(π·P·L))  eqn. 11

Switchable directional display apparatuses for use in privacy displayfor example and comprising plural retarders arranged between a displaypolariser and an additional polariser are described in U.S. Pat. No.10,126,575 and in U.S. patent application Ser. No. 16/131,419 titled“Optical stack for switchable directional display” (Attorney DocketNumber 412101), filed Sep. 14, 2018, both of which are hereinincorporated by reference in their entireties. Directional displayapparatuses further comprising reflective polarisers arranged betweenthe display polariser and retarders are described in U.S. Patent Publ.No. 2018-0329245, which is herein incorporated by reference in itsentirety. Directional display polarisers comprising passive retardersarranged between a display polariser and an additional polariser aredescribed in U.S. Patent Publ. No. 2018-0321553, which is hereinincorporated by reference in its entirety.

The structure and operation of various switchable display devices willnow be described. In this description, common elements have commonreference numerals. It is noted that the disclosure relating to anyelement applies to each device in which the same or correspondingelement is provided. Accordingly, for brevity such disclosure is notrepeated.

FIG. 1A is a schematic diagram illustrating in side perspective view anoptical stack of a display device for use in ambient illumination; FIG.1B is a schematic diagram illustrating in side perspective view aswitchable privacy display for use in ambient illumination comprising anemissive spatial light modulator (SLM) and compensated switchableretarder, and FIG. 2A is a schematic diagram illustrating in front viewalignment of optical layers in the optical stack of FIG. 1.

A display device 100 for use in ambient illumination 604 comprises: aSLM 48 arranged to output light 400; wherein the SLM 48 comprises anoutput polariser 218 arranged on the output side of the SLM 48, theoutput polariser 218 being a linear polariser; an additional polariser318 arranged on the output side of the output polariser 218, theadditional polariser 318 being a linear polariser: and a reflectivepolariser 302 arranged between the output polariser 218 and theadditional polariser 318, the reflective polariser 302 being a linearpolariser. Typical polarisers 210, 218, 318 may be polarisers such asdichroic polarisers.

At least one polar control retarder 300 is arranged between thereflective polariser 302 and the additional polariser 318. The electricvector transmission direction 303 of the reflective polariser 302 isparallel to the electric vector transmission direction 319 of theadditional polariser 318. The electric vector transmission direction 303of the reflective polariser 302 is parallel to the electric vectortransmission direction 219 of the output polariser 218.

Thus a display device for use in ambient illumination 604 comprises aSLM 48 arranged to output light 400. In the present disclosure, SLM 48may comprise a liquid crystal display comprising input polariser 210,output polariser 218 with substrates 212, 216, liquid crystal layer 214and red, green and blue pixels 220, 222, 224. Backlight 20 may bearranged to illuminate the SLM 48 and may comprise input light sources15, waveguide 1, rear reflector 3 and optical stack 5 comprisingdiffusers, light turning films and other known optical backlightstructures. Asymmetric diffusers, that may comprise asymmetric surfacerelief features for example, may be provided in the optical stack 5 withincreased diffusion in the elevation direction in comparison to thelateral direction may be provided. Advantageously image uniformity maybe increased.

The structure and operation of backlights 20 for use in privacy displayare further described with reference to FIGS. 30A-32C below. In anillustrative embodiment of FIG. 1A, the luminance at polar angles to thenormal to the SLM greater than 45 degrees may be at most 18%.

The display may further comprise a reflective recirculation polariser208 arranged between the backlight 20 and SLM 48. The reflectiverecirculation polariser 208 is different to the reflective polariser 302of the present embodiments. Reflective recirculation polariser 208provides reflection of polarised light from the backlight that has apolarisation that is orthogonal to the electric vector transmissiondirection of the dichroic input polariser 210. Reflective recirculationpolariser 208 does not reflect ambient light 604 to a snooper.

As illustrated in FIG. 1B, the SLM 48 may alternatively be provided byother display types that provide output light 400 by emission, such asorganic LED displays (OLED), with output polariser 218. Output polariser218 may provide reduction of luminance for light reflected from the OLEDpixel plane by means of one of more retarders 518 inserted between theoutput display polariser 218 and OLED pixel plane. The one or moreretarders 518 may be a quarter waveplate and is different to theretarder 330 of the present disclosure.

Thus the SLM 48 comprises an output polariser 218 arranged on the outputside of the SLM 48. The output polariser 218 may be arranged to providehigh extinction ratio for light from the pixels 220, 222, 224 of the SLM48 and to prevent back reflections from the reflective polariser 302towards the pixels 220, 222, 224.

Polar control retarder 300 is arranged between the reflective polariser302 and the additional polariser 318. In the embodiment of FIGS. 1A-1B,the polar control retarder 300 comprises passive polar control retarder330 and switchable liquid crystal retarder 301, but in general may bereplaced by other configurations of at least one retarder, some examplesof which are present in the devices described below.

The at least one polar control retarder 300 is capable of simultaneouslyintroducing no net relative phase shift to orthogonal polarisationcomponents of light passed by the reflective polariser 302 along an axisalong a normal to the plane of the at least one polar control retarder300 and introducing a relative phase shift to orthogonal polarisationcomponents of light passed by the reflective polariser 302 along an axisinclined to a normal to the plane of the at least one polar controlretarder 300. The polar control retarder 300 does not affect theluminance of light passing through the reflective polariser 302, thepolar control retarder 300 and the additional polariser 318 along anaxis along a normal to the plane of the polar control retarder 300 butthe polar control retarder 300 does reduce the luminance of lightpassing therethrough along an axis inclined to a normal to the plane ofthe polar control retarder 300, at least in one of the switchable statesof the switchable retarder 301. The principles leading to this effectare described in greater detail below with reference to FIGS. 33A-35Eand arises from the presence or absence of a phase shift introduced bythe polar control retarder 300 to light along axes that are angleddifferently with respect to the liquid crystal material of the polarcontrol retarder 300. A similar effect is achieved in all the devicesdescribed below.

Polar control retarder 300 comprises a switchable liquid crystalretarder 301 comprising a layer 314 of liquid crystal material, andsubstrates 312, 316 arranged between the reflective polariser 302 andthe additional polariser 318. Thus at least one polar control retarder300 comprises a switchable liquid crystal retarder 301 comprising alayer 314 of liquid crystal material 414, wherein the at least one polarcontrol retarder 300 is arranged, in a switchable state of theswitchable liquid crystal retarder 301, simultaneously to introduce nonet relative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser 302 along an axis along a normal tothe plane of the at least one polar control retarder 300 and tointroduce a net relative phase shift to orthogonal polarisationcomponents of light passed by the reflective polariser 302 along an axisinclined to a normal to the plane of the at least one polar controlretarder.

As illustrated in FIG. 2A in the case when the SLM 48 is a liquidcrystal display, the input electric vector transmission direction 211 atthe input polariser 210 provides an input polarisation component thatmay be transformed by the liquid crystal layer 214 to provide outputpolarisation component determined by the electric vector transmissiondirection 219 of the output polariser 218. The electric vectortransmission direction of the reflective polariser 302 is parallel tothe electric vector transmission direction of the output polariser 218.Further the electric vector transmission direction 303 of the reflectivepolariser 302 is parallel to the electric vector transmission direction319 of the additional polariser 318.

The substrates 312, 316 illustrated in FIG. 1A of the switchable liquidcrystal retarder 301 comprise electrodes 413, 415 (illustrated in FIG.3) arranged to provide a voltage across the layer 314 of liquid crystalmaterial 414. Control system 352 is arranged to control the voltageapplied by voltage driver 350 across the electrodes of the switchableliquid crystal retarder 301.

Polar control retarder 300 further comprises a passive polar controlretarder 330 as will be described further below. The at least one polarcontrol retarder 300 comprises at least one passive retarder 330 whichis arranged to introduce no net relative phase shift to orthogonalpolarisation components of light passed by the reflective polariser 302along an axis along a normal to the plane of the at least one passiveretarder and to introduce a net relative phase shift to orthogonalpolarisation components of light passed by the reflective polariser 302along an axis inclined to a normal to the plane of the at least onepassive retarder.

Passive polar control retarder 330 may comprise retardation layer with asolid birefringent material 430, while switchable liquid crystalretarder 301 may comprise a layer 314 of liquid crystal material 414, aswill be described below.

FIG. 2B is a schematic diagram illustrating in side perspective view aview angle control element 260 comprising a reflective polariser 302; apolar control retarder 300 comprising passive polar control retarder330, a switchable liquid crystal retarder 301; and an additionalpolariser. Features of the arrangement of FIG. 2B not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

The view angle control optical element 260 is for application to theoutput side of a display device for use in ambient illumination 604comprising a SLM 48 arranged to output light; wherein the SLM 48comprises an output polariser 218 arranged on the output side of the SLM48; the view angle control optical element 260 comprising an additionalpolariser 318; a reflective polariser 302 arranged between the outputpolariser 218 and the additional polariser 318 on application of theview angle control optical element 260 to the display device: and atleast one polar control retarder 300 arranged between the reflectivepolariser 302 and the additional polariser 318: wherein the at least onepolar control retarder 300 is capable of simultaneously introducing nonet relative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser 302 along an axis along a normal tothe plane of the at least one polar control retarder 300 and introducinga relative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser 302 along an axis inclined to anormal to the plane of the at least one polar control retarder.

In use, view angle control optical element 260 may be attached by a useror may be factory fitted to a polarised output SLM 48. View anglecontrol optical element 260 may be provided as a flexible film forcurved and bent displays. Alternatively the view angle control opticalelement 260 may be provided on a rigid substrate such as a glasssubstrate.

Advantageously, an after-market privacy control element and/or straylight control element may be provided that does not require matching tothe panel pixel resolution to avoid Moiré artefacts. View angle controloptical element 260 may be further provided for factory fitting to SLM48.

By attaching the view angle control optical element 260 of FIG. 2B to anexisting display device, it is possible to form a display device asshown in any of FIGS. 1A-2A.

The arrangement and operation of the polar control retarder 300comprising a switchable liquid crystal retarder 301 will now bediscussed.

FIG. 3 is a schematic diagram illustrating in perspective side view anarrangement of the polar control retarder 300 in a privacy mode ofoperation comprising a negative C-plate passive polar control retarder330 and homeotropically aligned switchable liquid crystal retarder 301in a privacy mode of operation.

In FIG. 3 and other schematic diagrams below, some layers of the opticalstack are omitted for clarity. For example the switchable liquid crystalretarder 301 is shown omitting the substrates 312, 316. Features of thearrangement of FIG. 3 not discussed in further detail may be assumed tocorrespond to the features with equivalent reference numerals asdiscussed above, including any potential variations in the features.

The switchable liquid crystal retarder 301 comprises a layer 314 ofliquid crystal material 414 with a negative dielectric anisotropy. Thepassive polar control retarder 330 comprises a negative C-plate havingan optical axis perpendicular to the plane of the retarder 330,illustrated schematically by the orientation of the discotic material430.

The liquid crystal retarder 301 further comprises transmissiveelectrodes 413, 415 arranged to control the liquid crystal material, thelayer of liquid crystal material being switchable by means of adjustingthe voltage being applied to the electrodes. The electrodes 413, 415 maybe across the layer 314 and are arranged to apply a voltage forcontrolling the liquid crystal retarder 301. The transmissive electrodesare on opposite sides of the layer of liquid crystal material 414 andmay for example by ITO electrodes.

Alignment layers may be formed between electrodes 413, 415 and theliquid crystal material 414 of the layer 314. The orientation of theliquid crystal molecules in the x-y plane is determined by the pretiltdirection of the alignment layers so that each alignment layer has apretilt wherein the pretilt of each alignment layer has a pretiltdirection with a component 417 a, 417 b in the plane of the layer 314that is parallel or anti-parallel or orthogonal to the electric vectortransmission direction 303 of the reflective polariser 302.

Driver 350 provides a voltage V to electrodes 413, 415 across the layer314 of switchable liquid crystal material 414 such that liquid crystalmolecules are inclined at a tilt angle to the vertical, forming anO-plate. The plane of the tilt is determined by the pretilt direction ofalignment layers formed on the inner surfaces of substrates 312, 316.

In typical use for switching between a public mode and a privacy mode,the layer of liquid crystal material is switchable between two states,the first state being a public mode so that the display may be used bymultiple users, the second state being a privacy mode for use by aprimary user with minimal visibility by snoopers. The switching may beby means of a voltage being applied across the electrodes.

In general such a display may be considered having a first wide anglestate and a second reduced off-axis luminance state. Such a display mayprovide a privacy display. In another use or to provide controlledluminance to off-axis observers for example in an automotive environmentwhen a passenger or driver may wish some visibility of the displayedimage, without full obscuration, by means of intermediate voltagelevels. Stray light may be reduced for night-time operation.

The propagation of polarised light from the output polariser 218 willnow be considered for on-axis and off-axis directions.

FIG. 4A is a schematic diagram illustrating in side view propagation ofoutput light from a SLM through the optical stack of FIG. 1A in aprivacy mode of operation and FIG. 4B is a schematic graph illustratingthe variation of output luminance with polar direction for thetransmitted light rays in FIG. 4A. When the layer 314 of liquid crystalmaterial is in a second state of said two states, the polar controlretarder 300 provides no overall transformation of polarisationcomponent 360 to output light rays 400 passing therethrough along anaxis perpendicular to the plane of the switchable retarder, but providesan overall transformation of polarisation component 361 to light rays402 passing therethrough for some polar angles which are at an acuteangle to the perpendicular to the plane of the retarders. Features ofthe arrangement of FIG. 4A not discussed in further detail may beassumed to correspond to the features with equivalent reference numeralsas discussed above, including any potential variations in the features.

Polarisation component 360 from the output polariser 218 is transmittedby reflective polariser 302 and incident on retarders 300. On-axis lighthas a polarisation component 362 that is unmodified from component 360while off-axis light has a polarisation component 364 that istransformed by the polar control retarder 300. At a minimum, thepolarisation component 361 is transformed to a linear polarisationcomponent 364 and absorbed by additional polariser 318. More generally,the polarisation component 361 is transformed to an ellipticalpolarisation component, that is partially absorbed by additionalpolariser 318.

Thus in a polar representation of transmission by the polar controlretarder 300 and additional polariser 318 in a privacy mode, regions ofhigh transmission and regions of low transmission are provided asillustrated in FIG. 4B.

The polar distribution of light transmission illustrated in FIG. 4Bmodifies the polar distribution of luminance output of the underlyingSLM 48. In the case that the SLM 48 comprises a directional backlight 20then off-axis luminance may be further be reduced as described above.

Advantageously, a privacy display is provided that has low luminance toan off-axis snooper while maintaining high luminance for an on-axisobserver.

The operation of the reflective polariser 302 for light from ambientlight source 604 will now be described.

FIG. 5A is a schematic diagram illustrating in top view propagation ofambient illumination light through the optical stack of FIG. 1A in aprivacy mode of operation; and FIG. 5B is a schematic graph illustratingthe variation of reflectivity with polar direction for the reflectedlight rays in FIG. 5A. Features of the arrangement of FIG. 5A notdiscussed in further detail may be assumed to correspond to the featureswith equivalent reference numerals as discussed above, including anypotential variations in the features.

Ambient light source 604 illuminates the display 100 with unpolarisedlight. Additional polariser 318 transmits light ray 410 normal to thedisplay surface with a first polarisation component 372 that is a linearpolarisation component parallel to the electric vector transmissiondirection 319 of the additional polariser 318.

In both states of operation, the polarisation component 372 remainsunmodified by the polar control retarder 300 and so transmittedpolarisation component 382 is parallel to the transmission axis of thereflective polariser 302 and the output polariser 218, so ambient lightis directed through the SLM 48 and lost.

By comparison, for ray 412, off-axis light is directed through the polarcontrol retarder 300 such that polarisation component 374 incident onthe reflective polariser 302 may be reflected. Such polarisationcomponent is re-converted into component 376 after passing throughretarders 300 and is transmitted through the additional polariser 318.

Thus when the layer 314 of liquid crystal material is in the secondstate of said two states, the reflective polariser 302 provides noreflected light for ambient light rays 410 passing through theadditional polariser 318 and then the polar control retarder 300 alongan axis perpendicular to the plane of the polar control retarder 300,but provides reflected light rays 412 for ambient light passing throughthe additional polariser 318 and then the polar control retarder 300 atsome polar angles which are at an acute angle to the perpendicular tothe plane of the polar control retarder 300; wherein the reflected light412 passes back through the polar control retarder 300 and is thentransmitted by the additional polariser 318.

The polar control retarder 300 thus provides no overall transformationof polarisation component 380 to ambient light rays 410 passing throughthe additional polariser 318 and then the polar control retarder 300along an axis perpendicular to the plane of the switchable retarder, butprovides an overall transformation of polarisation component 372 toambient light rays 412 passing through the absorptive polariser 318 andthen the polar control retarder 300 at some polar angles which are at anacute angle to the perpendicular to the plane of the polar controlretarder 300.

The polar distribution of light reflection illustrated in FIG. 5B thusillustrates that high reflectivity can be provided at typical snooperlocations by means of the privacy state of the polar control retarder300. Thus, in the privacy mode of operation, the reflectivity foroff-axis viewing positions is increased, and the luminance for off-axislight from the SLM is reduced as illustrated in FIG. 4B.

Advantageously, a privacy display is provided that has high reflectivityto an off-axis snooper while maintaining low reflectivity for an on-axisobserver. As is described above, such increased reflectivity providesincreased visual security level for the display in an ambientlyilluminated environment.

In another application such a display may provide a switchable mirrorappearance. Such a display may improve the aesthetic appearance ofdisplays that are not in operation. For example in applications to atelevision in a domestic environment, the display may be provided as amirror for off-axis viewing, so hiding the ‘black hole’ that is typicalof large area TVs, by reflecting ambient light, advantageously providingperceived expansion of the living space.

Measurements of reflectivity of the arrangement of FIG. 5A will now bedescribed.

FIG. 5C is a schematic graph illustrating a measurement of the variationof reflectivity 390 with lateral viewing angle 392 for some reflectedlight rays 412. Profile 394 illustrates variation in reflectivity for adisplay in privacy mode, while profile 396 illustrates variation ofreflectivity for a display in public mode.

In comparison to FIG. 5B, the peak reflectivity is approximately 20%,where 50% represents the reflectivity of a perfect reflective polariser302. Such reduced reflectivity is due to transmission losses from theadditional polariser 318, reflective polariser polarisation reflectionefficiency, chromatic variation of the tuning point for the polarcontrol retarder 300 and other reflection and scatter losses within theoptical stack.

The operation of the privacy mode of the display of FIG. 1A will now bedescribed further.

FIG. 6A is a schematic diagram illustrating in front perspective viewobservation of transmitted output light for a display operating inprivacy mode. Display 100 may be provided with white regions 603 andblack regions 601. A snooper may observe an image on the display ifluminance difference between the observed regions 601, 603 can beperceived. In operation, primary user 45 observes a full luminanceimages by rays 400 to viewing locations 26 that may be optical windowsof a directional display. Snooper 47 observes reduced luminance rays 402in viewing locations 27 that may for example be optical windows of adirectional display comprising an imaging waveguide. Regions 26, 27further represent on-axis and off-axis regions of polar graphs 4B and5B.

FIG. 6B is a schematic diagram illustrating in front perspective viewobservation of reflected ambient light from interface surfaces of adisplay. Thus some light rays 404 illustrated in FIG. 5A may bereflected by the front surface of the additional polariser 318 and othersurfaces of the display. Typically, such reflectivity may be 4% for abonded optical stack at normal incidence and approximately 5% for abonded optical stack for 45 degrees incidence, due to Fresnelreflections at the air-polariser interface. Thus a low luminancereflected image 605 of source 604 may be observed by the snooper on thefront of the display 100.

FIG. 6C is a schematic diagram illustrating in front perspective viewobservation of reflected ambient light for the display of FIG. 1Aoperating in privacy mode. By way of comparison with FIG. 6B,substantially higher reflected luminance is observable from reflection606 of source 604. Features of the arrangement of FIGS. 6A-C notdiscussed in further detail may be assumed to correspond to the featureswith equivalent reference numerals as discussed above, including anypotential variations in the features.

The shape and distribution of the reflected image 606 is determined bythe ambient light source 604 spatial distribution but may be furtherdetermined by diffusion layers, particularly at the output surface ofthe additional polariser 318.

FIG. 7A is a schematic diagram illustrating in front perspective viewsthe appearance of the display of FIG. 1A operating in privacy mode 1with luminance and reflectivity variations as illustrated in FIG. 4B andFIG. 5B from different viewing positions. Thus each of the nine views520, 522, 524, 526, 528, 530, 532, 534 and 536 correspond to a view fromthe corresponding viewing position, as shown by the perspectives ofthose views.

Thus upper viewing quadrant views 530, 532, lower viewing quadrant views534, 536 and lateral viewing position views 526, 528 provide bothreduced luminance and increased reflections 606, 605 of ambient lightsource 604, whereas up/down central viewing region views 522, 524 andhead-on view 520 provides much higher luminance and low reflectivityregion 605, with substantially no visibility of reflection fromreflective polariser 302.

FIG. 7B is a schematic graph illustrating the variation of VisualSecurity Level 620 against the ratio 622 of ambient illuminance tohead-on luminance for an off-axis snooper of the switchable privacydisplay of FIG. 1A in a privacy mode of operation for arrangements forprofile 624 with the reflective polariser 302 and for profile 626 withno reflective polariser 302 and for the illustrative embodiment of TABLE1.

TABLE 1 Variation Variation 626 624 Snooper luminance/Head-on luminance0.5% Image Contrast 500:1 Head-on luminance/nits 200 Reflectivepolariser 302 & retarders 300 No Yes Total display reflectivity 5.0% 30%

FIG. 7B thus illustrates that advantageously visual security level isincreased by the reflective polariser 302.

In comparison to the present embodiments, omission of the reflectivepolariser 302 provides for visual security level, V that is less than4.0 for typical ambient illuminance. Such visual security levels do notachieve desirable privacy to snooper 27. The present embodiments achievehigh visual security levels above 4.0 for a lux/nit ratio of 20% orless. For example, desirable visual security may be achieved for ahead-on user 26 observing a 200 nit image in an environment with 40 nitambient illuminance. As ambient illuminance increases, the visualsecurity level increases.

FIG. 7C is a schematic graph illustrating the variation of visualsecurity level with polar direction for a display of FIG. 1A comprisinga collimated backlight 20 as will be described further below withrespect to FIGS. 32A-C and a ratio (lux/nit) of ambient illuminance(lux) to head-on luminance (nits) of 20%.

FIG. 7C illustrates a first polar region 690 for viewing by the primaryuser 26 wherein a visual security level, V of less than 1.2 is achieved,delivering an image visibility. W of greater than 83%. Advantageously,the display 100 may be conveniently seen with high contrast. In a secondpolar region 692, the visual security level, V is greater than 4.0 and asnooper's eye positioned in this region will not easily be able todiscern information on the display. Polar region 694 is intermediate theregions 690 and 692 and is a region of reduced image visibility althoughnot at desirable levels of visual security. Advantageously the presentembodiments achieve a large polar region 690 for the primary user andlarge polar region 692 for the snooper, and a small transition region694.

FIG. 7D is a schematic graph illustrating the variation of visualsecurity level with polar direction for a display comprising no pluralretarders for the same lux/nit ratio as FIG. 7C. In comparison to thepresent embodiments, the polar region 692 of desirable visual securitylevel V>4 is significantly reduced and the polar region 694 of reducedimage visibility but insufficient visual security level is increased.

By way of comparison with the present disclosure, single retarders thatprovide high reflectivity over a narrow angular range (such as‘bulls-eye’ patterns typical of single retarder layers, and describedfor example with reference to FIGS. 27A-B) do not achieve highreflectivity over a wide angular range. In particular, the double passof reflected light illustrated in FIG. 5A provides a very narrow regionof high reflectivity. The reflected light has to pass twice through theretarder with input and output ray directions inverted about the displaynormal. This multiplies the optical effect and confines highreflectivity to rays with elevation angles close to the design angle(for example +/−45 degrees lateral angle and zero degrees elevation).The underlying extended privacy performance about the horizontal of thepresent embodiments yield much larger regions of high visual securitye.g. polar region 692.

The present plural retarders of the present embodiments provide highreflectivity over a wide angular range and achieve desirable privacy toan off-axis snooper. Further the present retarders may be switched toprovide low reflectivity and high image visibility in a public mode ofoperation. Advantageously the plural retarders achieve significantlyincreased polar region 692 and significantly reduced polar region 694while achieving comfortable image visibility to the primary user inpolar region 690.

It may be desirable to provide controllable display illumination in anautomotive vehicle.

FIG. 8A is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 of an automotive vehicle 600 for both entertainmentand sharing modes of operation. Light cone 610 (for example representingthe cone of light within which the luminance is greater than 50% of thepeak luminance) may be provided by the luminance distribution of thedisplay 100 in the elevation direction and is not switchable. Furtherdisplay reflectivity may be increased compared to head-on reflectivityoutside this light cone 610.

FIG. 8B is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 in an entertainment mode of operation and operates ina similar manner to a privacy display. Light cone 612 is provided with anarrow angular range such that passenger 606 may see the display 100whereas driver 604 may not see an image on the display 100 as aconsequence of reduced luminance and increased reflectivity.Advantageously entertainment images may be displayed to the passenger606 without distraction to the driver 604.

FIG. 8C is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 in a sharing mode of operation. Light cone 614 isprovided with a wide angular range such that all occupants may perceivean image on the display 100, for example when the display is not inmotion or when non-distracting images are provided.

FIG. 8D is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 for both night-time and day-time modes of operation.In comparison to the arrangements of FIGS. 7C-E, the optical output isrotated so that the display elevation direction is along an axis betweenthe driver 604 and passenger 606 locations. Light cone 620 illuminatesboth driver 604 and passenger 606 and with low display reflectivity.

FIG. 8E is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 in a night-time mode of operation. Thus the displaymay provide a narrow angular output light cone 622. Stray light thatilluminates internal surfaces and occupants of the vehicle cabin 602 andcause distraction to driver 604 may advantageously be substantiallyreduced. Both driver 604 and passenger 606 may advantageously be able toobserve the displayed images.

FIG. 8F is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 in a day-time mode of operation. Thus the display mayprovide a narrow angular output light cone 624. Advantageously thedisplay may be conveniently observed by all cabin 602 occupants.

The displays 100 of FIGS. 8A-F may be arranged at other vehicle cabinlocations such as driver instrument displays, centre console displaysand seat-back displays.

The operation of the display device 100 in public mode representing afirst state will now be described and further details of the polarcontrol retarder 300 illustrated.

FIG. 9A is a schematic diagram illustrating in perspective side view anarrangement of the polar control retarder 300 in a public mode ofoperation. In the present embodiment, zero volts is provided across theliquid crystal retarder 301; and TABLE 2 describes an illustrativeembodiment for the arrangement of FIG. 9A. Features of the arrangementof FIG. 9A not discussed in further detail may be assumed to correspondto the features with equivalent reference numerals as discussed above,including any potential variations in the features.

TABLE 2 Passive polar control retarder(s) Active LC retarder Δn · d/Alignment Pretilt/ Δn · d/ FIG. Mode Type nm layers deg nm Δε Voltage/V9A, 9C, 9E Public Negative C −700 Homeotropic 88 810 −4.3 0 3, 4B, 5BPrivacy Homeotropic 88 2.2

The switchable liquid crystal retarder 301 comprises two surfacealignment layers disposed on electrodes 413, 415 and adjacent to thelayer of liquid crystal material 414 and on opposite sides thereof andeach arranged to provide homeotropic alignment in the adjacent liquidcrystal material 414. The layer of liquid crystal material 414 of theswitchable liquid crystal retarder 301 comprises a liquid crystalmaterial with a negative dielectric anisotropy. The liquid crystalmolecules 414 may be provided with a pretilt, for example 88 degreesfrom the horizontal to remove degeneracy in switching.

In the present embodiments, desirable ranges for retardations andvoltages have been established by means of simulation of retarder stacksand experiment with display optical stacks. Ranges for retardances willnow be described that provide design configurations for various opticallayers.

The layer 314 of liquid crystal material has a retardance for light of awavelength of 550 nm in a range from 500 nm to 1000 nm, preferably in arange from 600 nm to 900 nm and most preferably in a range from 700 nmto 850 nm; and the retarder 330 further comprises a passive retarderhaving an optical axis perpendicular to the plane of the retarder, thepassive retarder having a retardance for light of a wavelength of 550 nmin a range from −300 nm to −900 nm, preferably in a range from −450 nmto −800 nm and most preferably in a range from −500 nm to −725 nm.

Alternatively, the passive polar control retarder 330 may comprise anO-plate retarder having an optical axis that is oriented with acomponent perpendicular to the plane of the retarder and a component inthe plane of the retarder. Such a retarder may provide furthercompensation for residual tilts of the liquid crystal material 414.

FIG. 9B is a schematic diagram illustrating in side view propagation ofoutput light from a SLM through the optical stack of FIG. 1A in a publicmode of operation and FIG. 9C is a schematic graph illustrating thevariation of output luminance with polar direction for the transmittedlight rays in FIG. 9B. Features of the arrangements of FIGS. 9B-C notdiscussed in further detail may be assumed to correspond to the featureswith equivalent reference numerals as discussed above, including anypotential variations in the features.

Thus when the liquid crystal retarder 301 is in a first state of saidtwo states, the polar control retarder 300 provides no overalltransformation of polarisation component 360, 361 to output lightpassing therethrough perpendicular to the plane of the switchableretarder 301 or at an acute angle to the perpendicular to the plane ofthe switchable retarder 301. That is polarisation component 362 issubstantially the same as polarisation component 360 and polarisationcomponent 364 is substantially the same as polarisation component 361.Thus the angular transmission profile of FIG. 9C is substantiallyuniformly transmitting across a wide polar region. Advantageously adisplay may be switched to a wide field of view.

FIG. 9D is a schematic diagram illustrating in top view propagation ofambient illumination light through the optical stack of FIG. 1A in apublic mode of operation; and FIG. 9E is a schematic graph illustratingthe variation of reflectivity with polar direction for the reflectedlight rays in FIG. 9D. Features of the arrangements of FIG. 9D-E notdiscussed in further detail may be assumed to correspond to the featureswith equivalent reference numerals as discussed above, including anypotential variations in the features.

Thus when the liquid crystal retarder 301 is in the first state of saidtwo states, the polar control retarder 300 provides no overalltransformation of polarisation component 372 to ambient light rays 412passing through the additional polariser 318 and then the polar controlretarder 300, that is perpendicular to the plane of the polar controlretarder 300 or at an acute angle to the perpendicular to the plane ofthe polar control retarder 300.

In operation in the public mode, input light ray 412 has polarisationstate 372 after transmission through the additional polariser 318. Forboth head-on and off-axis directions no polarisation transformationoccurs and thus the reflectivity for light rays 402 from the reflectivepolariser 302 is low. Light ray 412 is transmitted by reflectivepolariser 302 and lost in the display polarisers 218, 210 or thebacklight of FIG. 1A or optical isolator 218, 518 in an emissive SLM 38of FIG. 1B.

Advantageously in a public mode of operation, high luminance and lowreflectivity is provided across a wide field of view. Such a display canbe conveniently viewed with high contrast by multiple observers.

The appearance of the display of FIG. 1A in public mode for the firststate will now be described.

FIG. 10A is a schematic diagram illustrating in front perspective viewobservation of transmitted output light for a display operating inpublic mode; FIG. 10B is a schematic diagram illustrating in frontperspective view observation of reflected ambient light from theswitchable display of FIG. 1A in public mode: and FIG. 10C is aschematic diagram illustrating in front perspective views the appearanceof the display of FIG. 1A operating in public mode.

Thus the desirable off-axis viewing position for user 49 has highdisplay luminance and substantially without reflections from thereflective polariser 302. A high image visibility value can be achievedand display information conveniently resolved by multiple users. Fresnelreflection 605 are still present as in conventional displays, and are ata customary low level. A high performance public mode is provided.

Further arrangements of retarders will now be described.

FIG. 11A is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a public mode of operationwherein the switchable retarder comprises a switchable liquid crystallayer with homogeneous alignment and crossed A-plate polar controlretarders; FIG. 11B is a schematic graph illustrating the variation ofoutput luminance with polar direction for transmitted light rays in FIG.11A in a privacy mode of operation; FIG. 11C is a schematic graphillustrating the variation in reflectivity with polar direction forreflected light rays in FIG. 11A in a privacy mode of operation: FIG.11D is a schematic graph illustrating the variation of output luminancewith polar direction for transmitted light rays in FIG. 11A in a publicmode of operation; and FIG. 11E is a schematic graph illustrating thevariation in reflectivity with polar direction for reflected light raysin FIG. 11A in a public mode of operation comprising the embodimentsillustrated in TABLE 3A. Features of the arrangements of FIGS. 11A-E notdiscussed in further detail may be assumed to correspond to the featureswith equivalent reference numerals as discussed above, including anypotential variations in the features.

TABLE 3A Passive polar control retarder(s) Active LC retarder Δn · d/Alignment Pretilt/ Δn · d/ FIG. Mode Type nm layers deg nm Δε Voltage/V11D, 11E Public Crossed A +500 @ 45° Homogeneous 2 750 13.2 10 11A, 11B,Privacy +500 @ 135° Homogeneous 2 2.3 11C

The switchable liquid crystal retarder 301 comprises two surfacealignment layers 419 a, 419 b disposed adjacent to the layer of liquidcrystal material 421 and on opposite sides thereof and each arranged toprovide homogeneous alignment in the adjacent liquid crystal material421. The layer 314 of liquid crystal material 421 of the switchableliquid crystal retarder 301 comprises a liquid crystal material 421 witha positive dielectric anisotropy. The layer of liquid crystal material421 has a retardance for light of a wavelength of 550 nm in a range from500 nm to 900 nm, preferably in a range from 600 nm to 850 nm and mostpreferably in a range from 700 nm to 800 nm. The retarder 330 furthercomprises a pair of passive retarders 330A, 330B which have optical axesin the plane of the retarders that are crossed, each passive retarder ofthe pair of passive retarders having a retardance for light of awavelength of 550 nm in a range from 300 nm to 800 nm, preferably in arange from 350 nm to 650 nm and most preferably in a range from 450 nmto 550 nm.

In comparison to the embodiments of TABLE 2, the passive polar controlretarder 330 is provided by a pair of A-plates 330A, 330B that havecrossed axes. In the present embodiments, ‘crossed’ refers to an angleof substantially 90° between the optical axes of the two retarders inthe plane of the retarders. To reduce cost of retarder materials, it isdesirable to provide materials with some variation of retarderorientation due to stretching errors during film manufacture forexample. Variations in retarder orientation away from preferabledirections can reduce the head-on luminance and increase the minimumtransmission. Preferably the angle 310A is at least 35° and at most 55°,more preferably at least 40° and at most 50° and most preferably atleast 42.5° and at most 47.5°. Preferably the angle 310B is at least125° and at most 145°, more preferably at least 130° and at most 135°and most preferably at least 132.5° and at most 137.5°.

In comparison to the embodiments of TABLE 2, the liquid crystal retarderalignment is provided by a homogeneous rather than homeotropicalignment. Homogeneous alignment advantageously provides reducedrecovery time during mechanical distortion, such as when touching thedisplay.

The passive retarders may be provided using stretched films toadvantageously achieve low cost and high uniformity. Further field ofview for liquid crystal retarders with homogeneous alignment isincreased while providing resilience to the visibility of flow of liquidcrystal material during applied pressure.

It may be desirable to provide the additional polariser 318 with adifferent electric vector transmission direction to the electric vectortransmission direction of the output polariser 218 and reflectivepolariser 302.

FIG. 11F is a schematic diagram illustrating in perspective side view anarrangement of retarders 300 in a privacy mode of operation comprisingthe crossed A-plate passive polar control retarders 330A, 330B andhomogeneously aligned switchable liquid crystal retarder 301, furthercomprising a passive rotation retarder 460 comprising the embodimentsillustrated in TABLE 3B. Features of the arrangement of FIG. 11F notdiscussed in further detail may be assumed to correspond to the featureswith equivalent reference numerals as discussed above, including anypotential variations in the features.

TABLE 3B Layer Orientation/° Retarder Retardance/nm Polariser 218 45 — —Reflective polariser 302 45 — — Rotation retarder 460 22.5 A-plate +275Polar control retarder 330A 45 A-plate +450 Polar control retarder 330A135 A-plate +450 Switchable LC 301 0 See TABLE 7 Polariser 318A 0 — —

The reflective polariser 302 and the additional polariser 318 haveelectric vector transmission directions 303, 319 that are not parallel,and the display device 100 further comprises a rotator retarder 406arranged between the reflective polariser 302 and the additionalpolariser 318, the rotator retarder 406 being arrange to rotate apolarisation direction of polarised light incident thereon between theelectric vector transmission direction of the display polariser 218 andelectric vector transmission direction of the additional polariser 318.

The output polariser 218 and reflective polariser 302 may be providedwith electric vector transmission directions 219, 303 that may be forexample at an angle 317 of 45 degrees in the case of a twisted nematicLCD display. The additional polariser 318 may be arranged to providevertically polarised light to a user who may be wearing polarisingsunglasses that typically transmit vertically polarised light.

The passive rotation retarder 460 is different to the polar controlretarder 330 of the present embodiments and its operation will now bedescribed. Passive rotation retarder 460 may comprise a birefringentmaterial 462 and be a half waveplate, with retardance at a wavelength of550 nm of 275 nm for example. Passive rotation retarder 460 has a fastaxis orientation 464 that is inclined at an angle 466 that may be 22.5degrees to the electric vector transmission direction 319 of theadditional polariser 318. The passive rotation retarder 460 thus rotatesthe polarisation from the output polariser 218 such that thepolarisation direction of the light that is incident onto the polarcontrol retarder 330B is parallel to the direction 319.

In operation the passive rotation retarder 460 modifies the on-axispolarisation state, by providing an angular rotation of the polarisationcomponent from the output polariser 218. In comparison to the polarcontrol retarders 330A, 330B together do not modify the on-axispolarisation state. Further, the passive rotation retarder 460 providesa rotation of polarisation that provides only a small variation ofoutput luminance with viewing angle for off-axis directions. Incomparison the polar control retarders 330A, 330B provide substantialmodifications of output luminance with viewing angle.

Advantageously a display may be provided with an output polarisationdirection 319 that is different from the display polariser polarisationdirection 219, for example to provide viewing with polarisingsunglasses.

In an alternative embodiment the separate retarder 460 may be omittedand the retardance of the retarder 330B of FIG. 11A increased to providethe half wave rotation. To continue the illustrative embodiment, theretardance of retarder 330B at a wavelength of 550 nm may be 275 nmgreater than the retardance of retarder 330A. Advantageously the numberof layers, complexity and cost may be reduced.

In other embodiments, the passive rotation retarder 460 may be providedbetween the display output polariser 218 and the reflective polariser302 such that the electric vector transmission directions 303, 319 ofthe reflective polariser 302 and additional polariser 318 are parallel.

FIG. 12A is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode of operationcomprising a homogeneously aligned switchable liquid crystal retardercomprising liquid crystal 421 and a passive negative C-plate retarder330 driven with a second voltage V1; and FIG. 12B is a schematic diagramillustrating in perspective side view an arrangement of a switchableretarder in a privacy mode of operation comprising a homogeneouslyaligned switchable liquid crystal retarder and a passive negativeC-plate retarder driven with a second voltage V2 different to the firstvoltage V1. Features of the arrangements of FIGS. 12A-B not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

In comparison to the arrangement of FIG. 12A, the drive voltage V2 isincreased to provide increased tilt for the molecules of the liquidcrystal material 414 in the centre of the layer 314 of the liquidcrystal retarder. Such increased tilt changes the retardation of theswitchable liquid crystal retarder 301 between the privacy and publicmodes.

FIG. 12C is a schematic graph illustrating the variation of outputluminance with polar direction for transmitted light rays in FIG. 12A ina privacy mode of operation; and FIG. 12D is a schematic graphillustrating the variation in reflectivity with polar direction forreflected light rays in FIG. 12A in a privacy mode of operation. FIG.12E is a schematic graph illustrating the variation of output luminancewith polar direction for transmitted light rays in FIG. 12B in a publicmode of operation; and FIG. 12F is a schematic graph illustrating thevariation in reflectivity with polar direction for reflected light raysin FIG. 12B in a public mode of operation. Illustrative embodiments ofthe arrangement of homogeneous alignment in combination with passiveretarders are shown in TABLE 4A.

TABLE 4A Passive polar control retarder(s) Active LC retarder Δn · d/Alignment Pretilt/ Δn · d/ FIG. Mode Type nm layers deg nm Δε Voltage/V12E, 12F Public Negative C −500 Homogeneous 2 750 +13.2 10.0 12C, 12DPrivacy Homogeneous 2 3.8

The switchable liquid crystal retarder 301 comprises two surfacealignment layers disposed adjacent to the layer of liquid crystalmaterial and on opposite sides thereof and each arranged to providehomogeneous alignment in the adjacent liquid crystal material 414. Thelayer 314 of liquid crystal material 414 of the switchable liquidcrystal retarder 301 comprises a liquid crystal material 414 with apositive dielectric anisotropy. The layer of liquid crystal material 414has a retardance for light of a wavelength of 550 nm in a range from 500nm to 900 nm, preferably in a range from 600 nm to 850 nm and mostpreferably in a range from 700 nm to 800 nm. The retarder 330 furthercomprises a passive retarder having an optical axis perpendicular to theplane of the retarder, the passive retarder having a retardance forlight of a wavelength of 550 nm in a range from −300 nm to −700 nm,preferably in a range from −350 nm to −600 nm and most preferably −400nm to −500 nm.

In comparison to FIG. 11A, advantageously thickness and complexity ofthe retarder 330 may be reduced.

A structure omitting passive polar control retarder 330 will now bedescribed.

FIG. 13A is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode of operationcomprising a homogeneously aligned switchable liquid crystal retarder301 in a privacy mode of operation. The switchable liquid crystalretarder 301 comprises surface alignment layers 419 a, 419 b disposedadjacent to the layer of liquid crystal material 421 and arranged toprovide homogeneous alignment at the adjacent liquid crystal material421. FIG. 13B is a schematic graph illustrating the variation of outputluminance with polar direction for transmitted light rays in FIG. 13A ina privacy mode of operation; FIG. 13C is a schematic graph illustratingthe variation in reflectivity with polar direction for reflected lightrays in FIG. 13A in a privacy mode of operation, FIG. 13D is a schematicgraph illustrating the variation of output luminance with polardirection for transmitted light rays in FIG. 13A in a public mode ofoperation; and FIG. 13E is a schematic graph illustrating the variationin reflectivity with polar direction for reflected light rays in FIG.13A in a public mode of operation.

An illustrative embodiment of the arrangement of FIG. 13A is given inTABLE 4B.

TABLE 4B Active LC retarder Alignment Pretilt/ Δn · d/ FIG. Mode layersdeg nm Δε Voltage/V 13D, 13E Public Homogeneous 1 900 +15 0 13A, 13B,Privacy Homogeneous 1 2.4 13C

The switchable liquid crystal retarder 301 comprises two surfacealignment layers disposed adjacent to the layer of liquid crystalmaterial and on opposite sides thereof and each arranged to providehomogeneous alignment in the adjacent liquid crystal material 414. Thelayer of liquid crystal material of the switchable liquid crystalretarder comprises a liquid crystal material with a positive dielectricanisotropy. The liquid crystal retarder 301 may have a retardance forlight of a wavelength of 550 nm in a range from 500 nm to 1500 nm,preferably in a range from 700 nm to 1200 nm and most preferably in arange from 800 nm to 000 nm.

The embodiments of FIGS. 13A-E advantageously achieve reduced cost andcomplexity as no passive retarder is provided. Further the public modemay be an undriven state of the liquid crystal material of the layer 314and a relatively low voltage is used in the privacy mode. Further, incomparison to homeotropic alignment, homogeneous alignment layers mayadvantageously provide reduced visibility of liquid crystal materialflow that arises from handling of the display surface, for example whena touch panel is used.

FIG. 13F is a schematic diagram illustrating in side perspective view aview angle control element comprising a reflective polariser, aswitchable liquid crystal retarder and an additional polariser. A lowcost switchable after-market layer may be provided that provides privacywith a switchable ‘bulls-eye’field-of-view profile.

Further arrangements of switchable retarders 300 will now be described.

FIG. 14A is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode of operationcomprising crossed A-plate passive retarders 330A, 330B andhomeotropically aligned switchable liquid crystal retarder 301. FIG. 14Bis a schematic graph illustrating the variation of output luminance withpolar direction for transmitted light rays in FIG. 14A in a privacy modeof operation: and FIG. 14C is a schematic graph illustrating thevariation in reflectivity with polar direction for reflected light raysin FIG. 14A in a privacy mode of operation. Features of the arrangementsof FIGS. 14A-C not discussed in further detail may be assumed tocorrespond to the features with equivalent reference numerals asdiscussed above, including any potential variations in the features.

FIG. 14D is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a public mode of operationcomprising crossed A-plate passive retarders and homeotropically alignedswitchable liquid crystal retarder. FIG. 14E is a schematic graphillustrating the variation of output luminance with polar direction fortransmitted light rays in FIG. 14D in a public mode of operation; andFIG. 14F is a schematic graph illustrating the variation in reflectivitywith polar direction for reflected light rays in FIG. 14D in a publicmode of operation. Features of the arrangements of FIGS. 14D-F notdiscussed in further detail may be assumed to correspond to the featureswith equivalent reference numerals as discussed above, including anypotential variations in the features.

Thus the passive polar control retarder 330 comprises a pair ofretarders 330A, 330B which have optical axes in the plane of theretarders that are crossed. The pair of retarders 330A, 330B haveoptical axes that each extend at +/−45° with respect to an electricvector transmission direction of the output polariser. The pair ofretarders 330A, 330B each comprise a single A-plate. An illustrativeembodiment is described in TABLE 5.

TABLE 5 Passive polar control retarder(s) Active LC retarder Δn · d/Alignment Pretilt/ Δn · d/ FIG. Mode Type nm layers deg nm Δε Voltage/V14D, 14E, 14F Public Crossed A +650 @ 45° Homeotropic 88 810 −4.3 0 14A,14B, 14C Privacy +650 @ −45° Homeotropic 88 2.3

The switchable liquid crystal retarder 301 comprises two surfacealignment layers disposed on electrodes 413, 415 and adjacent to thelayer of liquid crystal material 414 and on opposite sides thereof andeach arranged to provide homeotropic alignment in the adjacent liquidcrystal material 414. The layer of liquid crystal material 414 of theswitchable liquid crystal retarder 301 comprises a liquid crystalmaterial with a negative dielectric anisotropy. The layer 314 of liquidcrystal material has a retardance for light of a wavelength of 550 nm ina range from 500 nm to 1000 nm, preferably in a range from 600 nm to 900nm and most preferably in a range from 700 nm to 850 nm. The retarder301 further comprises a pair of passive retarders which have opticalaxes in the plane of the retarders that are crossed, each passiveretarder of the pair of passive retarders having a retardance for lightof a wavelength of 550 nm in a range from 300 nm to 800 nm, preferablyin a range from 500 nm to 700 nm and most preferably in a range from 550nm to 675 nm.

Advantageously high reflectivity may be provided over a wide field ofview in privacy mode. A-plates may be more conveniently manufactured atlower cost than for the C-plate retarders.

Hybrid aligned liquid crystal retarders 301 will now be described.

FIG. 15A is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode of operationcomprising a homogeneously and homeotropically aligned switchable liquidcrystal retarder 301 comprising liquid crystal material 423 and apassive negative C-plate retarder 330. FIG. 15B is a schematic graphillustrating the variation of output luminance with polar direction fortransmitted light rays in FIG. 15A in a privacy mode of operation; andFIG. 15C is a schematic graph illustrating the variation in reflectivitywith polar direction for reflected light rays in FIG. 15A in a privacymode of operation. FIG. 15D is a schematic graph illustrating thevariation of output luminance with polar direction for transmitted lightrays in FIG. 15A in a public mode of operation; and FIG. 15E is aschematic graph illustrating the variation in reflectivity with polardirection for reflected light rays in FIG. 15A in a public mode ofoperation. Features of the arrangements of FIGS. 15A-E not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

An embodiment of the arrangement of hybrid alignment comprising bothhomeotropic and homogeneous alignment layers in combination with apassive retarder, are illustrated in TABLE 6.

TABLE 6 Passive polar control retarder(s) Active LC retarder Δn · d/Alignment Pretilt/ Δn · d/ FIG. Mode Type nm layers deg nm Δε Voltage/V15D, 15E Public Negative C −1100 Homogeneous 2 1300 +4.3 15.0 15B, 15CPrivacy Homeotropic 88 2.8

The switchable liquid crystal retarder 301 comprises two surfacealignment layers 419 a, 419 b disposed adjacent to the layer 314 ofliquid crystal material 414 and on opposite sides thereof, one of thesurface alignment layers 419 a being arranged to provide homeotropicalignment in the adjacent liquid crystal material 414 and the other ofthe surface alignment layers 419 b being arranged to provide homogeneousalignment in the adjacent liquid crystal material 414.

In comparison to embodiments with two homeotropic or two homogeneousalignment layers, the design of passive polar control retarder 330 maybe different if placed on the side of the homeotropic alignment layer419 a or placed on the side of the homogeneous alignment layer 419 b.

When the surface alignment layer 419 b arranged to provide homogeneousalignment is between the layer 314 of liquid crystal material 414 andthe polar control retarder 330, the liquid crystal retarder 301 has aretardance for light of a wavelength of 550 nm in a range from 700 nm to2000 nm, preferably in a range from 1000 nm to 1500 nm and mostpreferably in a range from 1200 nm to 1500 nm. The polar controlretarder 300 may further comprise a passive polar control retarder 330having its optical axis perpendicular to the plane of the retarder, thepassive polar control retarder 330 having a retardance for light of awavelength of 550 nm in a range from −400 nm to −1800 nm, preferably ina range from −700 nm to −1500 nm and most preferably in a range from−900 nm to −1300 nm.

The C-plate of FIG. 15A may be replaced by crossed A-plates. When thepolar control retarder 300 further comprises a pair of passive retarderswhich have optical axes in the plane of the retarders that are crossed,each retarder of the pair of retarders having a retardance for light ofa wavelength of 550 nm in a range from 400 nm to 1800 nm, preferably ina range from 700 nm to 1500 nm and most preferably in a range from 900nm to 1300 nm.

When the surface alignment layer 419 a arranged to provide homeotropicalignment is between the layer 314 of liquid crystal material 414 andthe polar control retarder 330, the liquid crystal retarder 301 has aretardance for light of a wavelength of 550 nm in a range from 500 nm to1800 nm, preferably in a range from 700 nm to 1500 nm and mostpreferably in a range from 900 nm to 1350 nm. The polar control retarder300 may further comprise a passive polar control retarder 330 having itsoptical axis perpendicular to the plane of the retarder 330, the passivepolar control retarder 330 having a retardance for light of a wavelengthof 550 nm in a range from −300 nm to −1600 nm, preferably in a rangefrom −500 nm to −1300 nm and most preferably in a range from −700 nm to−1150 nm; or the retarder 330 may further comprise a pair of passiveretarders which have optical axes in the plane of the retarders that arecrossed, each retarder of the pair of retarders having a retardance forlight of a wavelength of 550 nm in a range from 400 nm to 1600 nm,preferably in a range from 600 nm to 1400 nm and most preferably in arange from 800 nm to 1300 nm.

Advantageously, hybrid alignment of FIG. 15A achieves increased polarangular range over which reflectivity from reflective polariser 302 isincreased.

Further display structures will now be described, comprising multipleoptical stacks to achieve control of field of view of a privacy or lowstray light display apparatus.

FIG. 16 is a schematic diagram illustrating in side perspective view aswitchable privacy display 100 for use in ambient illuminationcomprising a non-collimating backlight 20, a further passive polarcontrol retarder 300B arranged between a reflective recirculationpolariser 318B and the transmissive SLM 48, a reflective polariser 302,polar control retarder 300A and additional polariser 318A. Thus incomparison to the display of FIG. 1A, FIG. 16 further comprises afurther passive polar control retarder 300B arranged between the inputpolariser 210 of the transmissive SLM 48 and the further additionalpolariser 318B. A further additional polariser 318B is provided byreflective polariser 318B arranged to recirculate light in the backlight20 and advantageously increase efficiency in a similar manner to thereflective polariser 208 of FIG. 1A.

Advantageously the field of view of the display is modified by thefurther additional polariser 318B to reduce off-axis luminance from theSLM 48. Stray light is reduced and visual security level to a snooper isincreased. The additional polariser 318B may be a reflective polariser.This is different to reflective polariser 302. Additional reflectivepolariser 318B provides light recirculation in the backlight 20, anddoes not increase front reflection in privacy mode. Advantageouslyefficiency is increased.

FIG. 17A is a schematic diagram illustrating in side perspective view aswitchable privacy display for use in ambient illumination comprising anemissive SLM 48, a passive control retarder 300B, a further additionalpolariser 318B, a reflective polariser 302, plural retarders 300 and anadditional polariser 318A. A further polar control retarder 300B isarranged between the output polariser 218 and the reflective polariser302. A further additional polariser 318A is arranged between the furtherpolar control retarder 300B and the reflective polariser 302.

FIG. 17B is a schematic diagram illustrating in side perspective view aview angle control element 260 for an emissive display.

In operation, light from the display output polariser 218 has afield-of-view modification from the passive polar control retarder 300Band further additional polariser 318B. Advantageously, the field of viewfrom the emissive display is reduced. The reflective polariser 302,plural polar control retarders 300A and an additional polariser 318Aprovide switching between a public mode that is determined by the SLM48, retarder 300B and further additional polariser 318B and a privacymode with high off-axis reflectivity and reduced off-axis luminance incomparison to that achieved by the display 100 of FIG. 1B.

In comparison to the display of FIG. 1B, FIG. 17A further comprises afurther polar control retarder 300B and a further additional polariser318B, wherein the further polar control retarder 300B is arrangedbetween the first-mentioned additional polariser and the furtheradditional polariser 318.

It would be desirable to provide a public mode with high imagevisibility for off-axis viewing and a privacy mode with high visualsecurity level. Embodiments of switchable privacy displays comprisingfurther plural retarders and further additional polarisers will now bedescribed.

FIG. 18A is a schematic diagram illustrating in side perspective view aswitchable privacy display 100 for use in ambient illumination 604comprising a wide angle backlight 20 wherein first polar controlretarder 300A is arranged between the backlight 20 and the SLM 48 andfurther polar control retarder 300B is arranged to receive light fromthe SLM 48. Features of the arrangements of FIGS. 16-18B not discussedin further detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features.

FIG. 18A has a similar structure to FIG. 1A with view angle controlelement 260A provided to receive light from the output polariser 218 ofthe SLM.

By way of comparison, the backlight 20 may be provided by a wide anglebacklight 20, as described elsewhere rather than a directionalbacklight. The SLM 48 is a transmissive SLM arranged to receive outputlight 400 from the backlight 20, and the SLM 48 further comprises aninput polariser 210 arranged on the input side of the SLM 48, the inputpolariser 210 being a linear polariser. A further additional polariser318B is arranged on the input side of the input polariser 210, thefurther additional polariser 318B being a linear polariser. At least onefurther polar control retarder 300B is arranged between the furtheradditional polariser 318B and the input polariser 210.

The first-mentioned at least one polar control retarder 300A comprises afirst switchable liquid crystal retarder 301A comprising a first layer314A of liquid crystal material, and the at least one further polarcontrol retarder 300B comprises a second switchable liquid crystalretarder 301B comprising a second layer 314B of liquid crystal material.

Polar control retarder 300A comprises passive polar control retarder330A and switchable liquid crystal retarder 301A. Further polar controlretarder 300B comprises passive polar control retarder 330B andswitchable liquid crystal retarder 301B. The polar control retarder 300Bprovides a modification of output transmission polar luminance profileand the polar control retarder 300A provides a modification of outputtransmission polar luminance and reflectivity profiles as describedelsewhere herein.

In comparison to FIG. 16, increased off-axis luminance is achieved inpublic mode as the backlight 20 has higher luminance for off-axis polarlocations so the image visibility is increased for off-axis users bycontrol of both liquid crystal layers 314A, 314B. In privacy mode, thevisual security level is increased for off-axis snoopers because theoff-axis luminance is reduced by two multiplicative luminance controlpolar control retarder 300A. 300B and respective additional polarisers318A, 318B. Further high reflectivity is provided for off-axis users.

Advantageously the reflective recirculation polariser with operation asdescribed with reference to FIG. 1A (that is different in function toreflective polariser 302) may provide the further additional polariser318B, to further achieve high efficiency and reduced field of view forprivacy operation.

The arrangement of FIG. 18A has a single view angle control element 260Aon the front surface of the SLM 48. Advantageously the front of screenthickness may be reduced. Further diffusers may be arranged on the frontsurface of the polariser 318 or between the view angle control element260A and output polariser 218. Advantageously the visibility of frontsurface reflections may be reduced. Further the view angle controlelement 260B may be conveniently provided between the SLM 48 andbacklight 20. Cost and complexity of assembly may be reduced. The numberof surfaces between the pixel layer 214 and the display user 26 may bereduced advantageously achieving increased image contrast.

An arrangement similar to FIG. 18A wherein the passive retarders 330A,330B each comprise a pair of passive plural retarders will now bedescribed.

FIG. 18B is a schematic diagram illustrating in front view alignment ofoptical layers of an optical stack comprising polar control retarder300A arranged between a reflective polariser 302 and an additionalpolariser 318A and further polar control retarder 300B arranged betweenthe input polariser 210 and a further additional polariser 318B of atransmissive SLM 48 wherein the polar control retarder 300A and furtherpolar control retarder 300B each comprise crossed A-plates. Anillustrative embodiment is provided in TABLE 7 and TABLE 8A.

TABLE 7 Active LC retarder Alignment layers Pretilt/deg Δn · d/nm ΔεVoltage/V Homogeneous 2 600 16.4 10 Homogeneous 2 2.0

TABLE 8A Layer Orientation/° Retarder Retardance/nm Polariser 318B 90 —— Retarder 330BA 45 A-plate +450 Retarder 330BB 135 A-plate +450Switchable LC 314B 0 See TABLE 7 Polariser 210 90 — — Polariser 218 0 —— Reflective polariser 302 0 — — Retarder 330AA 135 A-plate +450Retarder 330AB 45 A-plate +450 Switchable LC 314A 0 See TABLE 7Polariser 318A 0 — —

In comparison to the embodiment of FIG. 11A, the embodiment of FIGS.18A-F comprise either a transmissive SLM 48 that has high luminance atwide field of view and a wide angle backlight 20, or an emissive SLM 48.

Further the embodiments include polar control retarder 300A, additionalpolariser 318A, further polar control retarder 300B and furtheradditional polariser 318B. The transmission profiles of sucharrangements are multiplicative. Thus, very low luminance may beachieved at design polar angles, such as at a lateral angle of +/−45degrees and elevation of 0 degrees. However, the high luminance from thebacklight or emissive SLM at higher angles than the design polar angleprovides increased light levels and reduced reflectivity. Visualsecurity level may be reduced for high angle snoopers. The at least onefurther polar control retarder 300B comprises at least one furtherpassive retarder, in the embodiment of FIG. 18B two crossed passivepolar control retarders 330BA, 330BB are provided.

It may be desirable to provide designs that are tuned for minima thatare at lateral angles greater than 45 degrees, for example between 50degrees and 65 degrees. In arrangements with further polar controlretarder 300B, the layers 314A, 314B of liquid crystal material 414 ofthe switchable liquid crystal retarders 301A, 301B may each have aretardance for light of a wavelength of 550 nm in a range from 450 nm to850 nm, preferably in a range from 500 nm to 750 nm and most preferablyin a range from 550 nm to 650 nm.

The first-mentioned plural retarders and the further plural retardersmay each comprise a pair of passive retarders which have optical axes inthe plane of the retarders that are crossed wherein each passiveretarder of the first-mentioned pair of passive retarders 330A, 330B hasa retardance for light of a wavelength of 550 nm in a range from 300 nmto 800 nm, preferably in a range from 350 nm to 650 nm and mostpreferably in a range from 400 nm to 550 nm.

Advantageously the luminance and reflectivity at high angles may bereduced and the visual security level may be increased for snoopers at ahigh viewing angle. The reduction of colour asymmetry in switchableprivacy display will now be discussed.

FIG. 18C is a schematic graph illustrating the variation of logarithmicoutput luminance with polar direction for transmitted light rays ofplural retarders comprising crossed passive A-plates and a homogeneouslyaligned switchable liquid crystal retarder for one of the polar controlretarder 300B comprising crossed A-plate retarders 330AA, 330AB andliquid crystal layer 314A of TABLE 8A, and FIG. 18D is a schematic graphillustrating in a lateral direction the variation 470A of logarithmicoutput luminance with lateral viewing angle for transmitted light raysof plural retarders comprising crossed passive A-plates and ahomogeneously aligned switchable liquid crystal retarder for one of thepolar control retarder 300B comprising retarders 330AA, 330AB, 314A ofTABLE 8B.

FIG. 18C and FIG. 18D illustrate that there is some luminance asymmetrythat is provided by the sequence of the crossed A-plates 330AA, 330AB.As the luminance profiles are wavelength dependent, in operation suchasymmetry may provide a noticeable colour change that has a differentappearance on either side of the display, as illustrated in angularregions 472L, 472R. In the arrangement of FIG. 11A, such colour shift isnot typically very visible due to the low luminance of the collimatedbacklight 20. However, with increased backlight 20 luminance at highangles, or for emissive SLMs then the asymmetry is more clearly visible.It would be desirable to provide an asymmetric colour appearance.

Returning to FIG. 18B, the first-mentioned polar control retarder 300Acomprises a pair of passive retarders 330AA, 330AB which have opticalaxes 331AA, 331AB in the plane of the retarders 330AA, 330AB that arecrossed, wherein the first of the pair of passive retarders 330AA has anoptical axis 331AA that extends at 45° with respect to an electricvector transmission direction 219 of the output polariser 218, and thesecond of the pair of passive retarders 331AB has an optical axis 331ABthat extends at 1350 with respect to the electric vector transmissiondirection 219 of the output polariser 218.

The at least one further polar control retarder 300B comprises a furtherpair of passive retarders 330BA, 330BB which have optical axes 331BA,331BB in the plane of the retarders 330BA, 330BB that are crossed,wherein the first of the further pair of passive retarders 330BA has anoptical axis 331BA that extends at 135° with respect to an electricvector transmission direction 219 of the output polariser 218, and thesecond of the further pair of passive retarders 330BB has an opticalaxis 331BB that extends at 45° with respect to an electric vectortransmission direction 219 of the output polariser 218. The secondretarder 330AB, 330BB of each pair of passive retarders is arranged toreceive light from the first retarder 330AA, 330BA of the respectivepair of passive polar control retarder 330A, 330B. Thus, the passiveretarder 330 AA of the first pair and the passive retarder 330BB of thefurther pair that are closest to each other have respective optical axes331AA and 331AB that extend in the same direction.

For the present disclosure the rotation direction of the passiveretarder optical axes may be clockwise or anti-clockwise, such thateither one of the optical axis within each pair of passive retardersextend at 45° and 135°, respectively. In the illustrative example therotation direction is clockwise.

Returning to FIG. 18D, the luminance profile of the crossed A-plates330BA, 330BB and liquid crystal retarder 314B is illustrated by profile470B. In combination, the profiles 470A, 470B are multiplicative. Thearrangement of retarders of FIG. 18B thus achieves an averaging of thetwo luminance profiles and further achieves colour symmetry.Advantageously angular uniformity is improved.

Optionally, in the example of FIGS. 18A and 18B, the reflectivepolariser 318B may be omitted. In that case, the reflective polariser318B may optionally be replaced by a dichroic absorbing polariser (notshown).

Another arrangement of polar control retarder 300A, and further polarcontrol retarder 300B will now be given.

FIG. 18E is a schematic diagram illustrating in side perspective view aswitchable privacy display for use in ambient illumination comprising anemissive SLM 48, a first polar control retarder 300A, a first additionalpolariser 318A, a reflective polariser 302, a second polar controlretarder 300B and a second additional polariser 318B: and FIG. 18F is aschematic diagram illustrating in front view alignment of optical layersof an optical stack comprising polar control retarders 300A arrangedbetween an additional light absorbing polariser 318A and a furtheradditional polariser 318B that is a reflective polariser 302 and furtherpolar control retarders 300B arranged between the output polariser 218and the further additional polariser 318B, 302 wherein the polar controlretarders 300A and further plural retarders 300B each comprise crossedA-plates 330AA, 330AB, 330BA and 330BB.

An illustrative embodiment is provided in TABLE 8B.

TABLE 8B Layer Orientation/° Retarder Retardance/nm Polariser 218 0 — —Retarder 330AA 45 A-plate +450 Retarder 330AB 135 A-plate +450Switchable LC 314A 0 See TABLE 7 Reflective polariser 302 0 — — Retarder330BA 135 A-plate +450 Retarder 330BB 45 A-plate +450 Switchable LC 314B0 See TABLE 7 Polariser 318B 0 — —

In comparison to the arrangement of FIG. 18A, reduced luminance foroff-axis viewing locations may be advantageously provided as scatterfrom the SLM 48 does not modify the field of view of the luminanceprofile from the view angle control element 260B. Further a singleoptical component stack may be provided for convenient after-market orfactory fitting.

The embodiment of FIGS. 18E-F further illustrate that the reflectivepolariser 302 may further provide the additional polariser 318B of thefurther plural retarders 300B. Advantageously the cost and thickness isreduced, and the efficiency is increased.

Embodiments of luminance controlling displays with symmetric colour andluminance output will now be described.

FIG. 18G is a schematic diagram illustrating in front view alignment ofoptical layers of an optical stack comprising polar control retarders300A arranged between an additional light absorbing polariser 318A and afurther additional polariser 318B and further polar control retarders300B arranged between the output polariser 218 and the furtheradditional polariser 318B wherein the polar control retarders 300A andfurther plural retarders 300B each comprise crossed A-plate retarders330AA, 330AB, 330BA and 330BB.

A display device thus comprises: a SLM 48; a display polariser arrangedon at least one side of the SLM, the display polariser being a linearpolariser; a first additional polariser 318A arranged on the same sideof the SLM 48 as one of the at least one display polarisers, the firstadditional polariser 318A being a linear polariser; and first polarcontrol retarders 300A arranged between the first additional polariser318A and the one of the at least one display polarisers; a furtheradditional polariser 318B arranged on the same side of the SLM as saidone of the at least one display polarisers, outside the first additionalpolariser 318A, the further additional polariser 318B being a linearpolariser; and further polar control retarders 300B arranged between thefirst additional polariser 318A and the further additional polariser318B, wherein the first polar control retarders comprise a pair ofpassive retarders 330AA, 330AB which have optical axes 331AA, 331AB inthe plane of the retarders that are crossed and extend at 45° and 135°,respectively, with respect to an electric vector transmission directionof the output polariser 218, the further polar control retarderscomprise a further pair of passive retarders 330BA, 330BB which haveoptical axes 331BA, 331BB in the plane of the retarders that are crossedand extend at 45° and 135°, respectively with respect to an electricvector transmission direction of the output polariser 218, and theoptical axes 331BB, 33 IAA of the one of the first pair of passive polarcontrol retarders and the one of the further pair of passive polarcontrol retarders that are closest to each other extend in the samedirection.

The first polar control retarders 300A and further polar controlretarders 300B each further comprise a switchable liquid crystalretarder 301A. 301B comprising a layer 314A, 314B of liquid crystalmaterial 414A, 414B, the first polar control retarders 300A and thefurther polar control retarders 300B each being arranged, in aswitchable state of the switchable liquid crystal retarder 301A, 301B,simultaneously to introduce no net relative phase shift to orthogonalpolarisation components of light passed by said one of the at least onedisplay polarisers along an axis along a normal to the plane of thepolar control retarders and to introduce a net relative phase shift toorthogonal polarisation components of light passed by said one of the atleast one display polarisers along an axis inclined to a normal to theplane of the polar control retarders.

The example of FIG. 18G is the same as that of FIGS. 18E and 18F, exceptthat the reflective polariser 302 is replaced by the further additionalpolariser 318B. Advantageously a luminance controlling display withsymmetric colour and luminance output in the lateral direction may beachieved, in the same manner as illustrated by FIGS. 18C-D. Further, thereflectivity of reflective polariser 302 is eliminated for environmentswhere high off-axis reflectivity is undesirable.

FIG. 18H is a schematic diagram illustrating in front view alignment ofoptical layers of an optical stack for a transmissive SLM 48 comprisingfurther plural retarders 300B arranged between a further additionallight absorbing polariser 318B and an additional polariser 318A andplural retarders 300A arranged between the input polariser 210 and theadditional polariser 318A wherein the plural retarders 300A and furtherplural retarders 300B each comprise crossed A-plates.

The example of FIG. 18H is the same as that of FIG. 18G, except that theoptical stack is arranged on the input side of a SLM and between thebacklight 20 and the SLM 48. Advantageously the front-of-screenthickness is reduced, and increased diffusion may be provided on thefront surface without blurring pixels. Further image contrast may beincreased.

FIG. 18I is a schematic diagram illustrating in front view alignment ofoptical layers of an optical stack for a transmissive SLM 48 comprisingfurther plural retarders 300B arranged between a further additionalpolariser 318B and the input polariser 210; and plural retarders 300Aarranged between the output polariser 218 and an additional polariser318A wherein the plural retarders 300A, 300B and further pluralretarders each comprise crossed A-plates 330AA, 330AB, 330BA, 330BB.

A display device comprises: a backlight 20 arranged to output light atransmissive SLM 48 arranged to receive output light from the backlight20; an input polariser 210 arranged on the input side of the SLM 48 andan output polariser 218 arranged on the output side of the SLM 48, theinput polariser 210 and the output polariser 218 being linearpolarisers; a first additional polariser 318A arranged on the outputside of output polariser 218, the first additional polariser 318A beinga linear polariser; and first polar control retarders 300A arrangedbetween the first additional polariser 318A and the output polariser218; a further additional polariser 318B arranged between the backlight20 and input polariser 210, the further additional polariser 318B beinga linear polariser; and further polar control retarders 300B arrangedbetween the input polariser 210 and the further additional polariser318B; wherein the first polar control retarders 300A comprise a pair ofpassive retarders 330AA, 330AB which have optical axes 331AA, 331AB inthe plane of the retarders that are crossed and extend at 45° and 135°,respectively, with respect to an electric vector transmission directionof the output polariser 218, the further polar control retarders 300Bcomprise a further pair of passive retarders 330BA, 330BB which haveoptical axes 331BA, 331BB in the plane of the retarders that are crossedand extend at 45° and 135°, respectively with respect to an electricvector transmission direction of the output polariser 218, and theoptical axes 331BB, 331AA of the one of the first pair of passive polarcontrol retarders and the one of the further pair of passive polarcontrol retarders that are closest to each other extend in the samedirection.

The example of FIG. 18I is the same as that of FIG. 18H, except that theoptical stack is arranged on both sides of a SLM 48 and between thebacklight 20 and the SLM 48. Advantageously scatter from the SLM doesnot provide stray light to the snooper and higher visual security levelmay be achieved.

FIG. 18J is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode of operationcomprising a negative C-plate passive polar control retarder 330A andhomogeneously aligned switchable liquid crystal retarder 301A arrangedbetween the output polariser 218 and reflective polariser 302 and anegative C-plate passive polar control retarder 330B and homogeneouslyaligned switchable liquid crystal retarder 301B arranged between theabsorptive polariser 318 and reflective polariser 302 in a privacy modeof operation. Thus the display device may further comprise a retardancecontrol layer 300A arranged between the output polariser 218 and thereflective polariser 302. The retardance control layer 300A may comprisea further switchable liquid crystal retarder 301A arranged between theoutput polariser 218 and the reflective polariser 302.

The first-mentioned polar control retarder 300B comprises a firstswitchable liquid crystal retarder 301B comprising a first layer ofliquid crystal material 414B, and the further polar control retarder300A comprises a second switchable liquid crystal retarder 301Acomprising a second layer of liquid crystal material 414A. The furtherswitchable liquid crystal retarder 301A comprises a surface alignmentlayer 307A disposed adjacent the liquid crystal material 414A having apretilt having a pretilt direction with a component in the plane of thelayer of liquid crystal material that is aligned parallel orantiparallel or orthogonal to the reflective polariser.

The pretilt directions 307A, 331A of the alignment layers of the furtherswitchable liquid crystal retarder 301A may have a component in theplane of the liquid crystal layer 314A that is aligned parallel orantiparallel or orthogonal to the pretilt directions of the alignmentlayers 307B. 331B of the first switchable liquid crystal retarder 301B.In a public mode of operation, both switchable liquid crystal layers301B, 301A are driven to provide a wide viewing angle. In a privacy modeof operation, switchable liquid crystal retarders 301A, 301B maycooperate to advantageously achieve increased luminance reduction andthus improved privacy in a single axis.

The first and second liquid crystal retarders 301A, 301B may haveretardances that are different. The retardation provided by the firstliquid crystal retarder 301B and further liquid crystal layer 314A maybe different. The control system 352 may be arranged to control apply acommon voltage across the first and second switchable liquid crystalretarders 301A, 301B. The liquid crystal material 414B of the firstliquid crystal retarder 301B may be different from the liquid crystalmaterial 414A of the second liquid crystal layer 301A. Chromaticvariation of the polar luminance profiles illustrated elsewhere hereinmay be reduced, so that advantageously off-axis colour appearance isimproved.

Alternatively, switchable liquid crystal retarders 301A, 301B may haveorthogonal alignments so that reduced luminance is achieved in bothhorizontal and vertical directions, to advantageously achieve landscapeand portrait privacy operation.

The retardance control layer 300A may comprise a passive polar controlretarder 330A arranged between the output polariser 218 and thereflective polariser 302. More generally, the switchable liquid crystalretarder 301A may be omitted and a fixed luminance reduction may beprovided by passive retarders 330A. For example, luminance reduction inviewing quadrants may be provided by means of layer 330A alone.Advantageously polar region for luminance reduction may be achieved.

FIG. 18J further illustrates that the reflective polariser 302 mayprovide the further additional polariser 318B and that the dichroicpolariser 318B of FIG. 18F for example may be omitted. Advantageouslyincreased efficiency and reduced thickness may be achieved.

FIG. 18K is a schematic diagram illustrating in side perspective view aview angle control element 260 comprising a first polar control retarder300A a first additional polariser 318A, a reflective polariser 302, asecond polar control retarder 300B and a second additional polariser318B. Advantageously, an after-market privacy control element and/orstray light control element may be provided that does not requirematching to the panel pixel resolution to avoid Moiré artefacts. Viewangle control optical element 260 may be further provided for factoryfitting to SLM 48. Features of the arrangements of FIGS. 18E-H notdiscussed in further detail may be assumed to correspond to the featureswith equivalent reference numerals as discussed above, including anypotential variations in the features.

It may be desirable to provide both entertainment and night-time modesof operation in an automotive vehicle.

FIG. 19A is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display such as that illustratedin FIGS. 19A-B arranged within the vehicle cabin 602 for day-time and/orsharing modes of operation; and FIG. 19B is a schematic diagramillustrating in side view an automotive vehicle with a switchabledirectional display arranged within the vehicle cabin 602 for day-timeand/or sharing modes of operation. Light cone 630, 632 is provided witha wide angular field of view and thus the display is advantageouslyvisible by multiple occupants with low reflectivity.

FIG. 19C is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display such as that illustratedin FIGS. 19A-B arranged within the vehicle cabin 602 for night-timeand/or entertainment modes of operation; FIG. 19D is a schematic diagramillustrating in side view an automotive vehicle with a switchabledirectional display arranged within the vehicle cabin 602 for night-timeand/or entertainment modes of operation. Light cone 634, 636 is providedwith a narrow angular field of view and thus the display isadvantageously visible only by a single occupant. Off-axis occupantsfurther see increased reflections from the display, reducing visibility.Advantageously stray light for night-time operation is reduced,increasing driver safety. Further, reflections of the display fromwindscreen 601 are reduced, minimising distraction to the driver 604.Features of the arrangements of FIGS. 19A-D not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

Displays 100 comprising polar control retarders 300 that are passiveretarders 272 and comprising reflective polariser 302 and additionalpolariser 318 will now be further described.

FIG. 20A is a schematic diagram illustrating in side perspective view aprivacy display 100 for use in ambient illumination 604 comprising abacklight 20, a transmissive SLM 48, a reflective polariser 302, passivepolar control retarders 300 comprising passive retarders 272A, 272B,272C and 272C: and additional polariser 318; and FIG. 20B is a schematicdiagram illustrating in side perspective view a view angle controlelement comprising a reflective polariser 302, passive polar controlretarders 300 and an additional polariser 318.

The operation of such a display is described below with reference toFIGS. 22A-B. Advantageously a low cost privacy or other type of lowstray light display may be provided. Further the complexity andthickness of the display is reduced in comparison to switchable displays100.

FIG. 20C is a schematic diagram illustrating in side perspective view aview angle control element 260 comprising passive polar controlretarders 300A comprising passive retarders 272AA, 272AB, 272AC, 272ADarranged between an additional polariser 318A and a reflective polariser302; and a further additional polariser 318B and further passive polarcontrol retarders 300B comprising passive retarders 272BA, 272BB, 272BC,272BD arranged on the input side of the reflective polariser 302. Theadditional polariser and additional In comparison to FIG. 20B,advantageously off-axis luminance may be further reduced while head-onluminance may be substantially maintained when attached to the output ofa SLM.

FIG. 20D is a schematic diagram illustrating in side perspective view aprivacy display for use in ambient illumination. Compared to FIG. 20A, afurther additional polariser 318B that is a reflective polariser, andfurther polar control retarders 300B comprising retarders 272BA, 272BBare arranged at the input to the SLM. The further additional polariser318B and 300B achieve increased luminance reduction for wide anglebacklights 20. Advantageously visual security level may be increased forwide angle backlights. In comparison to the switchable arrangementsdescribed elsewhere thickness and cost is reduced. Features of thearrangements of FIGS. 20A-D not discussed in further detail may beassumed to correspond to the features with equivalent reference numeralsas discussed above, including any potential variations in the features.

The embodiments of FIGS. 20A-D illustrate stack of passive polar controlretarders 300 that comprises four passive retarders as will beillustrated in FIGS. 22A-B below. However, other types of passiveretarder stacks will also be described below and may be incorporated.Various combinations of polar control retarders 300 comprising passiveretarders 272 arranged between a reflective polariser 302 and additionalpolariser 318 will now be described.

FIG. 21A is a schematic diagram illustrating in side perspective view anoptical stack of a passive retarder comprising a negative C-plate andarranged to provide field-of-view modification of a display device; andFIG. 21B is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in thepassive retarder of FIG. 21A.

FIG. 21C is a schematic diagram illustrating in side perspective view anoptical stack of a passive retarder comprising a negative O-plate tiltedin a plane orthogonal to the display polariser electric vectortransmission direction and a negative C-plate and arranged to providefield-of-view modification of a display device; and FIG. 21D is aschematic graph illustrating the variation of output transmission withpolar direction for transmitted light rays in the passive retarder ofFIG. 21C, comprising the structure illustrated in TABLE 9A.

TABLE 9A Passive retarder Out of plane In plane Δn · d/ FIGS. Layer Typeangle/° angle/° nm 21C & 21D 272A Negative O 65 90 −550 272B Positive C90 0 +500

The passive polar control retarder 300B thus comprises a passiveretarder 272A that is a negative O-plate which has an optical axis witha component in the plane of the passive retarder 272A and a componentperpendicular to the plane of the passive retarder 272A. Further thecomponent in the plane of the passive retarder extends at 90°, withrespect to an electric vector transmission direction that is parallel tothe electric vector transmission 219 of the display polariser 218. Thepassive retarder 272B comprises a passive retarder having an opticalaxis perpendicular to the plane of the passive retarder.

Advantageously luminance may be reduced for lateral viewing directions.A mobile display may be comfortably rotated about a horizontal axiswhile achieving privacy for off-axis snoopers in a lateral direction.

FIG. 21E is a schematic diagram illustrating in side perspective view anoptical stack of a passive retarder comprising crossed A-plates and apositive O-plate: and FIG. 21F is a schematic graph illustrating thevariation of output transmission with polar direction for transmittedlight rays in the passive retarder of FIG. 21E, comprising the structureillustrated in TABLE 9B.

TABLE 9B Passive retarder Out of plane In plane Δn · d/ FIGS. Layer Typeangle/° angle/° nm 21E & 21F 272A Positive A 0 45 +500 272B Positive A 0135 +500 272C Positive O 65 90 +550

The passive polar control retarder 300B thus comprises passive retarders272A, 272B that are crossed A-plates and retarder 272C which has anoptical axis with a component in the plane of the passive retarder 272Cand a component perpendicular to the plane of the passive retarder 272C.The component in the plane of the passive retarder extends at 90°, withrespect to an electric vector transmission direction that is parallel tothe electric vector transmission 219 of the display polariser 218.Advantageously luminance may be reduced for lateral viewing directions.A mobile display may be comfortably rotated about a horizontal axiswhile achieving privacy for off-axis snoopers in a lateral direction.

It may be desirable to provide reduction of luminance in both lateraland elevation directions.

FIG. 22A is a schematic diagram illustrating in side perspective view anoptical stack of a passive retarders 272A-D comprising two pairs ofcrossed A-plates: and FIG. 22B is a schematic graph illustrating thevariation of output transmission with polar direction for transmittedlight rays in the passive retarder of FIG. 22A, comprising the structureillustrated in TABLE 10. Features of the arrangements of FIGS. 22A-22Bnot discussed in further detail may be assumed to correspond to thefeatures with equivalent reference numerals as discussed above,including any potential variations in the features.

TABLE 10 Passive control retarder Out of plane In plane Δn · d/ FIGS.Layer Type angle/° angle/° nm 22A, 22B 272A Positive A 0 45 700 272B 90272C 0 272D 135

The retarder thus comprises a pair of passive retarders 272A, 272D whichhave optical axes in the plane of the retarders that are crossed. Thepair of retarders each comprise plural A-plates having respectiveoptical axes aligned at different angles from each other. The pair ofpassive retarders 272B, 272C have optical axes that each extend at 90°and 0°, respectively, with respect to an electric vector transmissiondirection that is parallel to the electric vector transmission 211 ofthe display polariser 210.

The pair of passive retarders 272A, 272D have optical axes that extendat 45° and at 135°, respectively, with respect to an electric vectortransmission direction 211 that is parallel to the electric vectortransmission of the display polariser 218 respectively.

The display further comprises an additional pair of passive retarders272B, 272C disposed between the first-mentioned pair of passiveretarders 272A, 272D and which have optical axes in the plane of theretarders that are crossed. The additional pair of passive retarders272B, 272C have optical axes that each extend at 0° and at 90°,respectively, with respect to an electric vector transmission direction211, 317 that is parallel to the electric vector transmission of thedisplay polariser 210, 316.

The retardance of each A-plate for light of a wavelength of 550 nm maybe in a range from 600 nm to 850 nm, preferably in a range from 650 nmto 730 nm, and most preferably in a range from 670 nm to 710 nm. Thecolour change of absorbed light from a central viewing location to anoff-axis viewing location may be advantageously reduced.

In further illustrative embodiments, preferably the angle 273A is atleast 40° and at most 50°, more preferably at least 42.5° and at most47.5° and most preferably at least 44° and at most 46°. Preferably theangle 273D is at least 130° and at most 140°, more preferably at least132.5° and at most 137.5° and most preferably at least 134° and at most136°.

In further illustrative embodiments, the inner retarder pair 272B, 272Cmay have looser tolerances than the outer retarder pair 272A, 272D.Preferably the angle 273B is at least −10° and at most 100, mostpreferably at least −5° and at most 50 and most preferably at least −2°and at most 2°. Preferably the angle 273C is at least 80° and at most100°, more preferably at least 85° and at most 95° and most preferablyat least 88° and at most 92°.

The present embodiment provides a transmission profile that has somerotational symmetry. Advantageously a privacy display may be providedwith reduced visibility of image from a wide field of view for lateralor elevated viewing positions of a snooper. Further, such an arrangementmay be used to achieve enhanced privacy operation for landscape andportrait operation of a mobile display. Such an arrangement may beprovided in a vehicle to reduce stray light to off-axis passengers, andalso to reduce light falling on windscreen and other glass surfaces inthe vehicle.

FIG. 23A-B are schematic diagrams illustrating in side views part of adisplay comprising a switchable compensated retarder and optical bondinglayers 380. Features of the arrangements of FIGS. 23A-B not discussed infurther detail may be assumed to correspond to the features withequivalent reference numerals as discussed above, including anypotential variations in the features. Optical bonding layers 380 may beprovided to laminate films and substrates, achieving increasedefficiency and reduced luminance at high viewing angles in privacy mode.Further an air gap 384 may be provided between the SLM 48 and the polarcontrol retarder 300. To reduce wetting of the two surfaces at the airgap 384, an anti-wetting surface 382 may be provided to at least one ofthe polar control retarder 300 or SLM 48.

The retarder 330 may be provided between the switchable liquid crystallayer 314 and SLM 48 as illustrated in FIG. 23B, or may be providedbetween the additional polariser 318 and switchable liquid crystal layer314 as illustrated in FIG. 23A. Substantially the same opticalperformance is provided in both systems other than for hybrid alignmentas described elsewhere herein. It would be desirable to provide reducedthickness and reduced total number of optical components.

FIG. 24A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy angle modeof operation comprising a homeotropically aligned switchable liquidcrystal retarder arranged between first and second C-plate passive polarcontrol retarders, FIG. 24B and FIG. 24C are schematic graphsillustrating the variation of output transmission with polar directionfor transmitted light rays in the optical stack of FIG. 24A in a publicmode and a privacy mode of operation respectively: and FIG. 24D is aschematic graph illustrating the variation in reflectivity with polardirection for reflected light rays in FIG. 24A in a privacy mode ofoperation, comprising the embodiments illustrated in TABLE 11. Featuresof the arrangements of FIGS. 24A-D not discussed in further detail maybe assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

TABLE 11 Passive polar control retarder(s) Active LC retarder Δn · d/Alignment Pretilt/ Δn · d/ FIG. Mode Type nm layers deg nm Δε Voltage/V24B Public Negative C, 330A −275 Homogeneous 2 750 13.2 5.0 24C & 24DPrivacy Negative C, 330B −275 Homogeneous 2 2.6 25D Public A-plate, 330A575 Homogeneous 2 750 13.2 5.0 25E Privacy A-plate, 330B 575 Homogeneous2 2.6

The passive polar control retarder 330 comprises first and secondC-plates 330A, 330B; and the switchable liquid crystal layer 314 isprovided between the first and second C-plates 330A, 330B. Theswitchable liquid crystal retarder comprises two surface alignmentlayers 419 a, 419 b disposed adjacent to the layer 314 of liquid crystalmaterial 414 and on opposite sides thereof and each arranged to providehomogeneous alignment in the adjacent liquid crystal material 414. Thelayer of liquid crystal material 414 of the switchable liquid crystalretarder comprises a liquid crystal material 414 with a negativepositive dielectric anisotropy.

The layer of liquid crystal material 314 has a retardance for light of awavelength of 550 nm in a range from 500 nm to 1000 nm, preferably in arange from 600 nm to 900 nm and most preferably in a range from 700 nmto 850 nm. The two passive retarders each comprises a passive retarderhaving an optical axis perpendicular to the plane of the retarder with atotal retardance for light of a wavelength of 550 nm in a range −300 nmto −700 nm, preferably in a range from −350 nm to −600 nm and mostpreferably −400 nm to −500 nm.

FIG. 25A is a schematic diagram illustrating in perspective side view adisplay comprising a switchable compensated retarder arranged betweenfirst and second C-plate passive polar control retarder substrates; andFIG. 25B is a schematic diagram illustrating in side view part of adisplay comprising a switchable compensated retarder arranged betweenfirst and second C-plate passive polar control retarder substrates.

The polar control retarder 300 comprises two passive retarders 330A,330B, and a switchable liquid crystal retarder 301 comprising a layer314 of liquid crystal material provided between the two passiveretarders 330A, 330B. The display device 100 further comprises atransmissive electrodes 413, 415 and liquid crystal surface alignmentlayers 409, 411 formed on a side of each of the two passive retarders330A, 330B adjacent the switchable liquid crystal retarder layer 314.The display device 100 further comprises first and second substratesbetween which the switchable liquid crystal retarder layer 314 isprovided, the first and second substrates each comprising one of the twopassive retarders 330A, 330B.

Thus the first C-plate 330A has a transparent electrode layer 415 andliquid crystal alignment layer 411 formed on one side and the secondC-plate 330B has a transparent electrode layer 413 and liquid crystalalignment layer 409 formed on one side.

The liquid crystal layer 314 is provided between first and secondsubstrates 312, 316, and the first and second substrates 312, 316 eachcomprises one of the first and second C-plates 330A, 330B. The C-platesmay be provided in double stretched COP films that are ITO coated toprovide electrodes 413, 415 and have liquid crystal alignment layers409, 411 formed thereon.

Advantageously, the number of layers may be reduced in comparison to thearrangement of FIG. 1, reducing thickness, cost and complexity. Furtherthe C-plates 330A, 330B may be flexible substrates, and may provide aflexible privacy display.

It would be desirable to provide a liquid crystal layer 314 betweenfirst and second A-plate substrates.

FIG. 25C is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a public mode ofoperation comprising a homogeneously aligned switchable liquid crystalretarder arranged between first and second crossed A-plate passive polarcontrol retarders; and FIG. 25D and FIG. 25E are schematic graphsillustrating the variation of output transmission with polar directionfor transmitted light rays for the structure of FIG. 25C when driven inwide angle and privacy modes of operation respectively comprising theembodiments further illustrated in TABLE 11. Features of thearrangements of FIGS. 25A-E not discussed in further detail may beassumed to correspond to the features with equivalent reference numeralsas discussed above, including any potential variations in the features.

The switchable liquid crystal retarder comprises two surface alignmentlayers 419 a, 419 b disposed adjacent to the layer 314 of liquid crystalmaterial 414 and on opposite sides thereof and each arranged to providehomogeneous alignment in the adjacent liquid crystal material 414. Thelayer of liquid crystal material 414 of the switchable liquid crystalretarder comprises a liquid crystal material 414 with a negativepositive dielectric anisotropy.

The layer of liquid crystal material 314 has a retardance for light of awavelength of 550 nm in a range from 500 nm to 1000 nm, preferably in arange from 600 nm to 900 nm and most preferably in a range from 700 nmto 850 nm. Each of the two passive retarders has an optical axis in theplane of the passive retarder, wherein the optical axes are crossed, andeach passive retarder of the pair of passive retarders having aretardance for light of a wavelength of 550 nm in a range from 150 nm to800 nm, preferably in a range from 200 nm to 700 nm and most preferablyin a range from 250 nm to 600 nm.

In comparison to the arrangement of FIG. 24A, advantageously A-platesmay be manufactured at reduced cost compared to C-plates.

It would be desirable to provide improved image appearance by means ofadding camouflage to the private image seen by the snooper 47 in privacymode of operation.

FIG. 26A is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode of operationcomprising a negative C-plate passive retarder and homeotropicallyaligned switchable liquid crystal retarder further comprising apatterned electrode 415 layer. At least one of the electrodes 413, 415may be patterned, in this example electrode 415 is patterned withregions 415 a, 415 b, 415 c and driven by respective voltage drivers 350a, 350 b, 350 c with voltages Va, Vb, Vc. Gaps 417 may be providedbetween the electrode regions 415 a, 415 b, 415 c. The tilt of themolecules 414 a, 414 b, 414 c may thus be adjusted independently toreveal a camouflage pattern with different luminance levels for off-axisviewing.

Thus the switchable liquid crystal retarder 301 arranged between thereflective polariser 302 and the additional polariser 318 is controlledby means of addressing electrodes 415 a, 415 b, 415 c and uniformelectrode 413. The addressing electrodes may be patterned to provide atleast two pattern regions comprising electrode 415 a and gap 417.

FIG. 26B is a schematic diagram illustrating in perspective front viewillumination of a primary viewer and a snooper by a camouflagedluminance controlled privacy display. Display 100 may have dark imagedata 601 and white background data 603 that is visible to the primaryviewer 45 in viewing window 26 p. By way of comparison snooper 47 maysee the camouflaged image as illustrated in FIG. 26C which is aschematic diagram illustrating in perspective side view illumination ofa snooper by a camouflaged luminance controlled privacy display.Features of the arrangements of FIGS. 26A-C not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

Thus in white background regions 603, a camouflage structure may beprovided that has mixed luminance of the white region 603. The patternregions of the electrodes 415 a, 415 b, 415 c are thus camouflagepatterns. At least one of the pattern regions is individuallyaddressable and is arranged to operate in a privacy mode of operation.

The pattern regions may be arranged to provide camouflage for multiplespatial frequencies by means of control of which patterns are providedduring privacy mode of operation. In an illustrative example, apresentation may be provided with 20 mm high text. A camouflage patternwith similar pattern size may be provided with a first control of anelectrode pattern. In a second example a photo may be provided withlarge area content that is most visible to a snooper 47. The spatialfrequency of the camouflage pattern may be reduced to hide the largerarea structures, by combining first and second electrode regions toprovide the voltage and achieve a resultant lower spatial frequencypattern.

Advantageously a controllable camouflage structure may be provided bymeans of adjustment of the voltages Va, Vb, Vc across the layer 892.Substantially no visibility of the camouflage structure may be seen forhead-on operation. Further the camouflage image may be removed byproviding Va, Vb and Vc to be the same.

Further to providing camouflage from luminance modulation of the privateimage, the present embodiments provide camouflaged reflection fromambient illumination 604, advantageously achieving further hiding ofprivate images to the snooper 47 while achieving non-camouflagedreflection to the primary user 45.

The performance of retarders between parallel polarisers when arrangedin series will now be described. First, the field of view of ahomogeneously aligned liquid crystal retarder 301 will now be describedfor two different drive voltages.

FIG. 27A is a schematic diagram illustrating in perspective side view anarrangement of a homogeneously aligned switchable liquid crystalretarder; FIG. 27B is a schematic graph illustrating the variation ofoutput transmission with polar direction for transmitted light rays inFIG. 27A for a first applied voltage; and FIG. 27C is a schematic graphillustrating the variation of output transmission with polar directionfor transmitted light rays in FIG. 27A for a second applied voltage thatis greater than the first applied voltage, comprising the structureillustrated in TABLE 12.

FIG. 27D is a schematic diagram illustrating in perspective side view aC-plate arranged between parallel polarisers; and FIG. 27E is aschematic graph illustrating the variation of output transmission withpolar direction for transmitted light rays in FIG. 27D, comprising thestructure illustrated in TABLE 12.

TABLE 12 Passive polar control retarder(s) Active LC retarder Δn · d/Central Alignment Pretilt/ Δn · d/ FIG. Type nm polariser? layers deg nmΔε Voltage/V 27A & 27B — — — Homogeneous 1 900 +15 2.4 27C Homogeneous20.0 27D & 27E Negative C −700 — — — — — — 28A & 28B Negative C −700 YesHomogeneous 1 900 +15 2.4 28C Homogeneous 20.0 29A & 29B Negative C −700No Homogeneous 1 900 +15 2.4 29C Homogeneous 20.0

FIG. 28A is a schematic diagram illustrating in perspective side view anarrangement of a homogeneously aligned switchable liquid crystalretarder 390 arranged between parallel polarisers 394, 396 in serieswith a field-of-view control passive retarder comprising a C-plateretarder 392 arranged between parallel polarisers 396, 398: FIG. 28B isa schematic graph illustrating the variation of output transmission withpolar direction for transmitted light rays in FIG. 28A for a firstapplied voltage; FIG. 28C is a schematic graph illustrating thevariation of output transmission with polar direction for transmittedlight rays in FIG. 28A for a second applied voltage that is greater thanthe first applied voltage, comprising the structure illustrated in TABLE12.

FIG. 29A is a schematic diagram illustrating in perspective side view anarrangement of a homogeneously aligned switchable liquid crystalretarder in series with a C-plate polar control retarder wherein thehomogeneously aligned switchable liquid crystal and C-plate polarcontrol retarder are arranged between a single pair of parallelpolarisers; FIG. 29B is a schematic graph illustrating the variation ofoutput transmission with polar direction for transmitted light rays inFIG. 29A for a first applied voltage; and FIG. 29C is a schematic graphillustrating the variation of output transmission with polar directionfor transmitted light rays in FIG. 29A for a second applied voltage thatis greater than the first applied voltage, comprising the structureillustrated in TABLE 12. Features of the arrangements of FIGS. 27A-29Cnot discussed in further detail may be assumed to correspond to thefeatures with equivalent reference numerals as discussed above,including any potential variations in the features.

Unexpectedly, the optimum conditions for maximum field-of-view operationis provided by equal and opposite net retardation of the polar controlretarder 330 in comparison to the switchable liquid crystal retarderlayer 314 in its undriven state. An ideal polar control retarder 330 andswitchable liquid crystal retarder layer 314 may achieve (i) nomodification of the public mode performance from the input light and(ii) optimal reduction of lateral viewing angle for off-axis positionsfor all elevations when arranged to provide a narrow angle state. Thisteaching may be applied to all the display devices disclosed herein.

It would be desirable to provide further reduction of off-axis luminanceby means of directional illumination from the SLM 48. Directionalillumination of the SLM 48 by directional backlights 20 will now bedescribed.

FIG. 30A is a schematic diagram illustrating in front perspective view adirectional backlight 20 (or ‘narrow angle’ or ‘collimated’ backlight),and FIG. 30B is a schematic diagram illustrating in front perspectiveview a non-directional backlight 20 (or ‘wide-angle’ backlight or‘non-collimated’ backlight), either of which may be applied in any ofthe devices described herein. Thus a directional backlight 20 as shownin FIG. 30A provides a narrow cone 450, whereas a non-directionalbacklight 20 as shown in FIG. 30B provides a wide angular distributioncone 452 of light output rays.

FIG. 30C is a schematic graph illustrating variation with luminance withlateral viewing angle for various different backlight arrangements. Thegraph of FIG. 30C may be a cross section through the polar field-of-viewprofiles described herein. Features of the arrangements of FIGS. 30A-Cnot discussed in further detail may be assumed to correspond to thefeatures with equivalent reference numerals as discussed above,including any potential variations in the features.

A Lambertian backlight has a luminance profile 846 that is independentof viewing angle. In the present embodiments, the backlight 20 may bearranged to provide an angular light distribution that has reducedluminance for off-axis viewing positions in comparison to head-onluminance.

A typical wide angle backlight has a roll-off at higher angles such thatthe full width half maximum of relative luminance may be preferablygreater than 40°, more preferably greater than 60° and most preferablygreater than 800. A typical wide angle backlight has a roll-off athigher angles such that the full width half maximum 866 of relativeluminance may be greater than 40°, preferably greater than 600 and mostpreferably greater than 800. Further the relative luminance 864 at+/−45°, is preferably greater than 7.5%, more preferably greater than10% and most preferably greater than 20%. Advantageously a display thatachieves a roll-off similar to the wide angle backlight may provide highimage visibility to off-axis users.

Displays comprising wide angle backlights 20 and only one additionalpolariser 318 and polar control retarder 330 (not comprising furtherpolar control retarders 300B and further additional polariser 318B) donot typically achieve desirable visual security level to off-axis usersin privacy mode of operation. Desirably such displays may be providedwith a directional backlight 20 as will now be described.

The backlight 20 may be a directional backlight that provides aluminance at polar angles to the normal to the SLM greater than 45degrees in at least one azimuthal direction that is at most 30% of theluminance along the normal to the SLM, preferably at most 20% of theluminance along the normal to the SLM, and more preferably at most 10%of the luminance along the normal to the SLM. The directional backlight20 may have a roll-off at higher angles such that the full width halfmaximum 862 of relative luminance may be less than 60°, preferably lessthan 400 and most preferably less than 200. In an illustrative examplethe luminance 868 at 45 degrees may be 18% of the head-on luminance fromthe backlight 20.

Such luminance profiles may be provided by the directional backlights 20described below or may also be provided by wide angle backlights incombination with further additional polariser 318B and polar controlretarders 300B as described elsewhere herein.

One type of a switchable backlight 20 will now be described.

FIG. 31A is a schematic diagram illustrating in side view a switchabledirectional display apparatus 100 comprising a switchable liquid crystalpolar control retarder 300 and backlight 20. The backlight 20 of FIG.31A may be applied in any of the devices described herein and whichcomprises an imaging waveguide 1 illuminated by a light source array 15through an input end 2. FIG. 31B which is a schematic diagramillustrating in rear perspective view operation of the imaging waveguide1 of FIG. 31A in a narrow angle mode of operation.

The imaging waveguides 1 is of the type described in U.S. Pat. No.9,519,153, which is herein incorporated by reference in its entirety.The waveguide 1 has an input end 2 extending in a lateral directionalong the waveguide 1. An array of light sources 15 are disposed alongthe input end 2 and input light into the waveguide 1.

The waveguide 1 also has opposed first and second guide surfaces 6, 8extending across the waveguide 1 from the input end 2 to a reflectiveend 4 for guiding light input at the input end 2 forwards and back alongthe waveguide 1. The second guide surface 8 has a plurality of lightextraction features 12 facing the reflective end 4 and arranged todeflect at least some of the light guided back through the waveguide 1from the reflective end 4 from different input positions across theinput end 2 in different directions through the first guide surface 6that are dependent on the input position.

In operation, light rays are directed from light source array 15 throughan input end and are guided between first and second guiding surfaces 6,8 without loss to a reflective end 4. Reflected rays are incident ontofacets 12 and output by reflection as light rays 230 or transmitted aslight rays 232. Transmitted light rays 232 are directed back through thewaveguide 1 by facets 803, 805 of rear reflector 800. Operation of rearreflectors are described further in U.S. Pat. No. 10,054,732, which isherein incorporated by reference in its entirety.

As illustrated in FIG. 31B, optical power of the curved reflective end 4and facets 12 provide an optical window 26 that is transmitted throughthe SLM 48 and has an axis 197 that is typically aligned to the opticalaxis 199 of the waveguide 1. Similar optical window 26 is provided bytransmitted light rays 232 that are reflected by the rear reflector 800.

FIG. 31C is a schematic graph illustrating field-of-view luminance plotof the output of FIG. 31B when used in a display apparatus with noswitchable liquid crystal retarder. Features of the arrangements ofFIGS. 31A-C not discussed in further detail may be assumed to correspondto the features with equivalent reference numerals as discussed above,including any potential variations in the features.

Thus for off-axis viewing positions observed by snoopers 47 may havereduced luminance, for example between 1% and 3% of the central peakluminance at an elevation of 0 degrees and lateral angle of +/−45degrees. Further reduction of off-axis luminance is achieved by theplural retarders 301, 330 of the present embodiments.

Backlight 20 may thus further comprise a switchable backlight arrangedto switch the output angular luminance profile in order to providereduced off-axis luminance in a privacy mode of operation and higheroff-axis luminance in a public mode of operation.

Another type of directional backlight with low off-axis luminance willnow be described.

FIG. 32A is a schematic diagram illustrating a side view a switchabledirectional display apparatus comprising a backlight 20 including aswitchable collimating waveguide 901 and a switchable liquid crystalpolar control retarder 300 and additional polariser 318. The backlight20 of FIG. 32A may be applied in any of the devices described herein andis arranged as follows.

The waveguide 901 has an input end 902 extending in a lateral directionalong the waveguide 901. An array of light sources 915 are disposedalong the input end 902 and input light into the waveguide 1. Thewaveguide 901 also has opposed first and second guide surfaces 906, 908extending across the waveguide 1 from the input end 2 to a reflectiveend 4 for guiding light input at the input end 2 forwards and back alongthe waveguide 1. In operation, light is guided between the first andsecond guiding surface 906, 908.

The first guiding surface 906 may be provided with a lenticularstructure 904 comprising a plurality of elongate lenticular elements 905and the second guiding surface 908 may be provided with prismaticstructures 912 which are inclined and act as light extraction features.The plurality of elongate lenticular elements 905 of the lenticularstructure 904 and the plurality of inclined light extraction featuresdeflect input light guided through the waveguide 901 to exit through thefirst guide surface 906.

A rear reflector 903 that may be a planar reflector is provided todirect light that is transmitted through the surface 908 back throughthe waveguide 901.

Output light rays that are incident on both the prismatic structures 912and lenticular elements 905 of the lenticular structure 904 are outputat angles close to grazing incidence to the surface 906. A prismaticturning film 926 comprising facets 927 is arranged to redirect outputlight rays 234 by total internal reflection through the SLM 48 andcompensated switchable liquid crystal polar control retarder 300.

FIG. 32B is a schematic diagram illustrating in top view output of thecollimating waveguide 901. Prismatic structures 912 are arranged toprovide light at angles of incidence onto the lenticular structure 904that are below the critical angle and thus may escape. On incidence atthe edges of a lenticular surface, the inclination of the surfaceprovides a light deflection for escaping rays and provides a collimatingeffect. Light ray 234 may be provided by light rays 188 a-c and lightrays 189 a-c, with incidence on locations 185 of the lenticularstructure 904 of the collimated waveguide 901.

FIG. 32C is a schematic graph illustrating an iso-luminancefield-of-view polar plot for the display apparatus of FIG. 32A. Thus anarrow output light cone may be provided, with size determined by thestructures of the structures 904, 912 and the turning film 926. Featuresof the arrangements of FIGS. 32A-C not discussed in further detail maybe assumed to correspond to the features with equivalent referencenumerals as discussed above, including any potential variations in thefeatures.

Advantageously in regions in which snoopers may be located with lateralangles of 45 degrees or greater for example, the luminance of outputfrom the display is small, typically less than 2%. It would be desirableto achieve further reduction of output luminance. Such further reductionis provided by the compensated switchable liquid crystal polar controlretarder 300 and additional polariser 318 as illustrated in FIG. 32A.Advantageously a high performance privacy display with low off-axisluminance may be provided over a wide field of view.

Directional backlights such as the types described in FIG. 31A and FIG.32A together with the plural retarders 301, 330 of the presentembodiments may achieve off-axis luminance of less than 1.5%, preferablyless than 0.75% and most preferably less than 0.5% may be achieved fortypical snooper 47 locations. Further, high on-axis luminance anduniformity may be provided for the primary user 45. Advantageously ahigh performance privacy display with low off-axis luminance may beprovided over a wide field of view, that may be switched to a publicmode by means of control of the switchable retarder 301 by means ofcontrol system 352 illustrated in FIG. 1A.

The operation of polar control retarder layers between parallelpolarisers for off-axis illumination will now be described further. Inthe various devices described above, at least one polar control retarderis arranged between the reflective polariser 318 and the additionalpolariser 218 in various different configurations. In each case, the atleast one polar control retarder is configured so that it does notaffect the luminance of light passing through the reflective polariser318, the at least one polar control retarder, and the additionalpolariser 218 along an axis along a normal to the plane of the polarcontrol retarder(s) but it does reduce the luminance of light passingthrough the reflective polariser 318, the at least one polar controlretarder, and the additional polariser 218 along an axis inclined to anormal to the plane of the polar control retarder(s), at least in one ofthe switchable states of the compensated switchable polar control polarcontrol retarder 300. There will now be given a description of thiseffect in more detail, the principles of which may be applied in generalto all of the devices described above.

FIG. 33A is a schematic diagram illustrating in perspective viewillumination of a polar control retarder layer by off-axis light. Polarcontrol retarder 630 may comprise birefringent material, represented byrefractive index ellipsoid 632 with optical axis direction 634 at 0degrees to the x-axis, and have a thickness 631. Features of thearrangements of FIGS. 33A-35E below that are not discussed in furtherdetail may be assumed to correspond to the features with equivalentreference numerals as discussed above, including any potentialvariations in the features.

Normal light rays 636 propagate so that the path length in the materialis the same as the thickness 631. Light rays 637 are in the y-z planehave an increased path length; however the birefringence of the materialis substantially the same as the rays 636. By way of comparison lightrays 638 that are in the x-z plane have an increased path length in thebirefringent material and further the birefringence is different to thenormal ray 636.

The retardance of the polar control retarder 630 is thus dependent onthe angle of incidence of the respective ray, and also the plane ofincidence, that is rays 638 in the x-z will have a retardance differentfrom the normal rays 636 and the rays 637 in the y-z plane.

The interaction of polarized light with the polar control retarder 630will now be described. To distinguish from the first and secondpolarization components during operation in a directional backlight 101,the following explanation will refer to third and fourth polarizationcomponents.

FIG. 33B is a schematic diagram illustrating in perspective viewillumination of a polar control retarder layer by off-axis light of athird linear polarization state at 90 degrees to the x-axis and FIG. 33Cis a schematic diagram illustrating in perspective view illumination ofa polar control retarder layer by off-axis light of a fourth linearpolarization state at 0 degrees to the x-axis. In such arrangements, theincident linear polarization states are aligned to the optical axes ofthe birefringent material, represented by ellipse 632. Consequently, nophase difference between the third and fourth orthogonal polarizationcomponents is provided, and there is no resultant change of thepolarization state of the linearly polarized input for each ray 636,637, 638. Thus, the polar control retarder 630 introduces no phase shiftto polarisation components of light passed by the polariser on the inputside of the polar control retarder 630 along an axis along a normal tothe plane of the polar control retarder 630. Accordingly, the polarcontrol retarder 630 does not affect the luminance of light passingthrough the polar control retarder 630 and polarisers (not shown) oneach side of the polar control retarder 630. Although FIGS. 29A-C relatespecifically to the polar control retarder 630 that is passive, asimilar effect is achieved by the polar control retarders in the devicesdescribed above.

FIG. 33D is a schematic diagram illustrating in perspective viewillumination of a polar control retarder 630 layer by off-axis light ofa linear polarization state at 45 degrees. The linear polarization statemay be resolved into third and fourth polarization components that arerespectively orthogonal and parallel to optical axis 634 direction. Thepolar control retarder thickness 631 and material retardance representedby refractive index ellipsoid 632 may provide a net effect of relativelyshifting the phase of the third and fourth polarization componentsincident thereon in a normal direction represented by ray 636 by half awavelength, for a design wavelength. The design wavelength may forexample be in the range of 500 to 550 nm.

At the design wavelength and for light propagating normally along ray636 then the output polarization may be rotated by 90 degrees to alinear polarization state 640 at −45 degrees. Light propagating alongray 637 may see a phase difference that is similar but not identical tothe phase difference along ray 637 due to the change in thickness, andthus an elliptical polarization state 639 may be output which may have amajor axis similar to the linear polarization axis of the output lightfor ray 636.

By way of contrast, the phase difference for the incident linearpolarization state along ray 638 may be significantly different, inparticular a lower phase difference may be provided. Such phasedifference may provide an output polarization state 644 that issubstantially circular at a given inclination angle 642. Thus, the polarcontrol retarder 630 introduces a phase shift to polarisation componentsof light passed by the polariser on the input side of the polar controlretarder 630 along an axis corresponding to ray 638 that is inclined toa normal to the plane of the polar control retarder 630. Although FIG.29D relates to the polar control retarder 630 that is passive, a similareffect is achieved by the polar control retarders described above, in aswitchable state of the switchable liquid crystal polar control retardercorresponding to the privacy mode.

To illustrate the off-axis behaviour of polar control retarder stacks,the angular luminance control of C-plates 330A, 330B between anadditional polariser 318 and output display polariser 218 will now bedescribed for various off-axis illumination arrangements with referenceto the operation of a C-plate between the parallel polarisers 500, 210will now be described.

FIG. 34A is a schematic diagram illustrating in perspective viewillumination of a C-plate layer by off-axis polarised light with apositive elevation. Incident linear polarisation component 704 isincident onto the birefringent material 632 of the polar controlretarder 560 that is a C-plate with optical axis direction 507 that isperpendicular to the plane of the polar control retarder 560.Polarisation component 704 sees no net phase difference on transmissionthrough the liquid crystal molecule and so the output polarisationcomponent is the same as component 704. Thus a maximum transmission isseen through the polariser 210. Thus the polar control retarder 560having an optical axis 561 perpendicular to the plane of the polarcontrol retarder 560, that is the x-y plane. The polar control retarder560 having an optical axis perpendicular to the plane of the polarcontrol retarder comprises a C-plate.

FIG. 34B is a schematic diagram illustrating in perspective viewillumination of a C-plate layer by off-axis polarised light with anegative lateral angle. As with the arrangement of FIG. 34A,polarisation state 704 sees no net phase difference and is transmittedwith maximum luminance. Thus, the polar control retarder 560 introducesno phase shift to polarisation components of light passed by thepolariser on the input side of the polar control retarder 560 along anaxis along a normal to the plane of the polar control retarder 560.Accordingly, the polar control retarder 560 does not affect theluminance of light passing through the polar control retarder 560 andpolarisers (not shown) on each side of the polar control retarder 560.Although FIGS. 29A-C relate specifically to the polar control retarder560 that is passive, a similar effect is achieved by the polar controlretarders in the devices described above.

FIG. 34C is a schematic diagram illustrating in perspective viewillumination of a C-plate layer by off-axis polarised light with apositive elevation and negative lateral angle. In comparison to thearrangement of FIGS. 34A-B, the polarisation state 704 resolves ontoeigenstates 703, 705 with respect to the birefringent material 632providing a net phase difference on transmission through the polarcontrol retarder 560. The resultant elliptical polarisation component656 is transmitted through polariser 210 with reduced luminance incomparison to the rays illustrated in FIGS. 34A-B.

FIG. 34D is a schematic diagram illustrating in perspective viewillumination of a C-plate layer by off-axis polarised light with apositive elevation and positive lateral angle. In a similar manner toFIG. 34C, the polarisation component 704 is resolved into eigenstates703, 705 that undergo a net phase difference, and ellipticalpolarisation component 660 is provided, which after transmission throughthe polariser reduces the luminance of the respective off-axis ray.Thus, the polar control retarder 560 introduces a phase shift topolarisation components of light passed by the polariser on the inputside of the polar control retarder 560 along an axis that is inclined toa normal to the plane of the polar control retarder 560. Although FIG.29D relates to the polar control retarder 560 that is passive, a similareffect is achieved by the polar control retarders described above, in aswitchable state of the switchable liquid crystal polar control retardercorresponding to the privacy mode.

FIG. 34E is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIGS.34A-D. Thus, the C-plate may provide luminance reduction in polarquadrants. In combination with switchable liquid crystal layer 314described elsewhere herein, (i) removal of luminance reduction of theC-plate may be provided in a first wide angle state of operation (ii)extended polar region for luminance reduction may be achieved in asecond privacy state of operation.

To illustrate the off-axis behaviour of polar control retarder stacks,the angular luminance control of crossed A-plates 330A, 330B between anadditional polariser 318 and output display polariser 218 will now bedescribed for various off-axis illumination arrangements.

FIG. 35A is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation. Linear polariser 218 with electricvector transmission direction 219 is used to provide a linearpolarisation state 704 that is parallel to the lateral direction ontofirst A-plate 330A of the crossed A-plates 330A, 330B. The optical axisdirection 331A is inclined at +45 degrees to the lateral direction. Theretardance of the polar control retarder 330A for the off-axis angle 60in the positive elevation direction provides a resultant polarisationcomponent 650 that is generally elliptical on output. Polarisationcomponent 650 is incident onto the second A-plate 330B of the crossedA-plates 330A, 330B that has an optical axis direction 331B that isorthogonal to the optical axis direction 331A of the first A-plate 330A.In the plane of incidence of FIG. 35A, the retardance of the secondA-plate 330B for the off-axis angle θ₁ is equal and opposite to theretardance of the first A-plate 330A. Thus a net zero retardation isprovided for the incident polarisation component 704 and the outputpolarisation component is the same as the input polarisation component704.

The output polarisation component is aligned to the electric vectortransmission direction of the additional polariser 318, and thus istransmitted efficiently. Advantageously substantially no losses areprovided for light rays that have zero lateral angle angular componentso that full transmission efficiency is achieved.

FIG. 35B is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a negative lateral angle. Thus input polarisation componentis converted by the first A-plate 330A to an intermediate polarisationcomponent 652 that is generally an elliptical polarisation state. Thesecond A-plate 330B again provides an equal and opposite retardation tothe first A-plate so that the output polarisation component is the sameas the input polarisation component 704 and light is efficientlytransmitted through the polariser 318.

Thus the polar control retarder comprises a pair of retarders 330A, 330Bwhich have optical axes in the plane of the retarders 330A, 330B thatare crossed, that is the x-y plane in the present embodiments. The pairof retarders 330A, 330B have optical axes 331A, 331B that each extend at45° with respect to an electric vector transmission direction that isparallel to the electric vector transmission of the polariser 318.

Advantageously substantially no losses are provided for light rays thathave zero elevation angular component so that full transmissionefficiency is achieved.

FIG. 35C is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation and negative lateral angle. Polarisationcomponent 704 is converted to an elliptical polarisation component 654by first A-plate 330A. A resultant elliptical component 656 is outputfrom the second A-plate 330B. Elliptical component 656 is analysed byinput polariser 318 with reduced luminance in comparison to the inputluminance of the first polarisation component 704.

FIG. 35D is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation and positive lateral angle. Polarisationcomponents 658 and 660 are provided by first and second A-plates 330A,330B as net retardance of first and second retarders does not providecompensation.

Thus luminance is reduced for light rays that have non-zero lateralangle and non-zero elevation components. Advantageously display privacycan be increased for snoopers that are arranged in viewing quadrantswhile luminous efficiency for primary display users is not substantiallyreduced.

FIG. 35E is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIGS.35A-D. In comparison to the arrangement of FIG. 34E, the area ofluminance reduction is increased for off-axis viewing. However, theswitchable liquid crystal layer 314 may provide reduced uniformity incomparison to the C-plate arrangements for off-axis viewing in the firstpublic mode state of operation.

As may be used herein, the terms “substantially” and “approximately”provide an industry-accepted tolerance for its corresponding term and/orrelativity between items. Such an industry-accepted tolerance rangesfrom zero percent to ten percent and corresponds to, but is not limitedto, component values, angles, et cetera. Such relativity between itemsranges between approximately zero percent to ten percent.

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of this disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theembodiment(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to anyembodiment(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the embodiment(s) set forth inissued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple embodimentsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theembodiment(s), and their equivalents, that are protected thereby. In allinstances, the scope of such claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

1. A display device for use in ambient illumination comprising: aspatial light modulator (SLM) arranged to output light; wherein the SLMcomprises an output polariser arranged on the output side of the SLM,the output polariser being a linear polariser; an additional polariserarranged on the output side of the output polariser, the additionalpolariser being a linear polariser; a reflective polariser arrangedbetween the output polariser and the additional polariser, thereflective polariser being a linear polariser; and at least one polarcontrol retarder arranged between the reflective polariser and theadditional polariser, wherein the at least one polar control retarder iscapable of simultaneously introducing no net relative phase shift toorthogonal polarisation components of light passed by the reflectivepolariser along an axis along a normal to the plane of the at least onepolar control retarder and introducing a relative phase shift toorthogonal polarisation components of light passed by the reflectivepolariser along an axis inclined to a normal to the plane of the atleast one polar control retarder.
 2. A display device according to claim1, wherein the at least one polar control retarder comprises aswitchable liquid crystal retarder comprising a layer of liquid crystalmaterial, wherein the at least one polar control retarder is arranged,in a switchable state of the switchable liquid crystal retarder,simultaneously to introduce no net relative phase shift to orthogonalpolarisation components of light passed by the reflective polariseralong an axis along a normal to the plane of the at least one polarcontrol retarder and to introduce a net relative phase shift toorthogonal polarisation components of light passed by the reflectivepolariser along an axis inclined to a normal to the plane of the atleast one polar control retarder.
 3. A display device according to claim2, wherein: the at least one polar control retarder further comprises atleast one passive retarder which is arranged to introduce no netrelative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser along an axis along a normal to theplane of the at least one passive retarder and to introduce a netrelative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser along an axis inclined to a normal tothe plane of the at least one passive retarder.
 4. A display deviceaccording to claim 2, wherein the switchable liquid crystal retardercomprises two surface alignment layers disposed adjacent to the liquidcrystal material on opposite sides thereof and each arranged to providehomeotropic alignment at the adjacent liquid crystal material.
 5. Adisplay device according to claim 4, wherein the layer of liquid crystalmaterial of the switchable liquid crystal retarder comprises a liquidcrystal material with a negative dielectric anisotropy.
 6. A displaydevice according to claim 4, wherein the layer of liquid crystalmaterial has a retardance for light of a wavelength of 550 nm in a rangefrom 500 nm to 1000 nm, preferably in a range from 600 nm to 900 nm andmost preferably in a range from 700 nm to 850 nm.
 7. A display deviceaccording to claim 4, wherein: the at least one passive retardercomprises a passive retarder having an optical axis perpendicular to theplane of the retarder, the passive retarder having a retardance forlight of a wavelength of 550 nm in a range from −300 nm to −900 nm,preferably in a range from −450 nm to −800 nm and most preferably in arange from −500 nm to −725 nm; or the at least one passive retardercomprises a pair of passive retarders which have optical axes in theplane of the retarders that are crossed, each passive retarder of thepair of passive retarders having a retardance for light of a wavelengthof 550 nm in a range from 300 nm to 800 nm, preferably in a range from500 nm to 700 nm and most preferably in a range from 550 nm to 675 nm.8. A display device according to claim 2, wherein the switchable liquidcrystal retarder comprises two surface alignment layers disposedadjacent to the layer of liquid crystal material and on opposite sidesthereof and each arranged to provide homogeneous alignment in theadjacent liquid crystal material.
 9. A display device according to claim8, wherein the layer of liquid crystal material of the switchable liquidcrystal retarder comprises a liquid crystal material with a positivedielectric anisotropy.
 10. A display device according to claim 8,wherein the layer of liquid crystal material has a retardance for lightof a wavelength of 550 nm in a range from 500 nm to 900 nm, preferablyin a range from 600 nm to 850 nm and most preferably in a range from 700nm to 800 nm.
 11. A display device according to claim 8, wherein: the atleast one passive retarder comprises a passive retarder having anoptical axis perpendicular to the plane of the retarder, the passiveretarder having a retardance for light of a wavelength of 550 nm in arange from −300 nm to −700 nm, preferably in a range from −350 nm to−600 nm and most preferably in a range from −400 nm to −500 nm; or theat least one passive retarder comprises a pair of passive retarderswhich have optical axes in the plane of the retarders that are crossed,each passive retarder of the pair of passive retarders having aretardance for light of a wavelength of 550 nm in a range from 300 nm to800 nm, preferably in a range from 350 nm to 650 nm and most preferablyin a range from 450 nm to 550 nm.
 12. A display device according toclaim 2, wherein the switchable liquid crystal retarder comprises twosurface alignment layers disposed adjacent to the layer of liquidcrystal material and on opposite sides thereof, one of the surfacealignment layers being arranged to provide homeotropic alignment in theadjacent liquid crystal material and the other of the surface alignmentlayers being arranged to provide homogeneous alignment in the adjacentliquid crystal material.
 13. A display device according to claim 12,wherein the surface alignment layer arranged to provide homogeneousalignment is between the layer of liquid crystal material and the polarcontrol retarder; the layer of liquid crystal material has a retardancefor light of a wavelength of 550 nm in a range from 700 nm to 2000 nm,preferably in a range from 1000 nm to 1500 nm and most preferably in arange from 1200 nm to 1500 nm; and the at least one passive retardercomprises a passive retarder having its optical axis perpendicular tothe plane of the retarder, the at least one passive retarder having aretardance for light of a wavelength of 550 nm in a range from −400 nmto −1800 nm, preferably in a range from −700 nm to −1500 nm and mostpreferably in a range from −900 nm to −1300 nm; or the at least onepassive retarder comprises a pair of passive retarders which haveoptical axes in the plane of the retarders that are crossed, eachretarder of the pair of retarders having a retardance for light of awavelength of 550 nm in a range from 400 nm to 1800 nm, preferably in arange from 700 nm to 1500 nm and most preferably in a range from 900 nmto 1300 nm.
 14. A display device according to claim 12, wherein thesurface alignment layer arranged to provide homeotropic alignment isbetween the layer of liquid crystal material and the polar controlretarder; the layer of liquid crystal material has a retardance forlight of a wavelength of 550 nm in a range from 500 nm to 1800 nm,preferably in a range from 700 nm to 1500 nm and most preferably in arange from 900 nm to 1350 nm; and the at least one passive retardercomprises a passive retarder having its optical axis perpendicular tothe plane of the retarder, the at least one passive retarder having aretardance for light of a wavelength of 550 nm in a range from −300 nmto −1600 nm, preferably in a range from −500 nm to −1300 nm and mostpreferably in a range from −700 nm to −1150 nm; or the at least onepassive retarder comprises a pair of passive retarders which haveoptical axes in the plane of the retarders that are crossed, eachretarder of the pair of retarders having a retardance for light of awavelength of 550 nm in a range from 400 nm to 1600 nm, preferably in arange from 600 nm to 1400 nm and most preferably in a range from 800 nmto 1300 nm.
 15. A display device according to claim 2, wherein eachalignment layer has a pretilt having a pretilt direction with acomponent in the plane of the layer of liquid crystal material that isparallel or anti-parallel or orthogonal to the electric vectortransmission direction of the reflective polariser.
 16. A display deviceaccording to claim 2, wherein the at least one polar control retarderfurther comprises two passive retarders, the switchable liquid crystalretarder being provided between the two passive retarders.
 17. A displaydevice according to claim 16, further comprising a transmissiveelectrode and liquid crystal surface alignment layer formed on a side ofeach of the two passive retarders adjacent the switchable liquid crystalretarder.
 18. A display device according to claim 16, further comprisingfirst and second substrates between which the switchable liquid crystalretarder is provided, the first and second substrates each comprisingone of the two passive retarders.
 19. A display device according toclaim 16, wherein the two passive retarders each comprises a passiveretarder having an optical axis perpendicular to the plane of theretarder with a total retardance for light of a wavelength of 550 nm ina range −300 nm to −700 nm, preferably in a range from −350 nm to −600nm and most preferably in a range from −400 nm to −500 nm.
 20. A displaydevice according to claim 16, wherein each of the two passive retardershas an optical axis in the plane of the passive retarder, wherein theoptical axes are crossed, and each passive retarder of the pair ofpassive retarders having a retardance for light of a wavelength of 550nm in a range from 150 nm to 800 nm, preferably in a range from 200 nmto 700 nm and most preferably in a range from 250 nm to 600 nm.
 21. Adisplay device according to claim 2, wherein the switchable liquidcrystal retarder further comprises transmissive electrodes arranged toapply a voltage for controlling the layer of liquid crystal material.22. A display device according to claim 21, wherein the transmissiveelectrodes are on opposite sides of the layer of liquid crystalmaterial.
 23. A display device according to claim 21, wherein theelectrodes are patterned to provide at least two pattern regions.
 24. Adisplay device according to claim 21, further comprising a controlsystem arranged to control the voltage applied across the electrodes ofthe switchable liquid crystal retarder.
 25. A display device accordingto claim 1, wherein the at least one polar control retarder comprises atleast one passive retarder which is arranged to introduce no netrelative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser along an axis along a normal to theplane of the at least one passive retarder and to introduce a netrelative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser along an axis inclined to a normal tothe plane of the at least one passive retarder.
 26. A display deviceaccording to claim 25, wherein the at least one passive retardercomprise at least two passive retarders with at least two differentorientations of optical axes.
 27. A display device according to claim25, wherein the at least one passive retarder comprise a passiveretarder having an optical axis perpendicular to the plane of theretarder.
 28. A display device according to claim 25, wherein the atleast one passive retarder comprises a pair of passive retarders whichhave optical axes in the plane of the retarders that are crossed.
 29. Adisplay device according to claim 28, wherein the pair of retarders haveoptical axes that extend at 45° and at 135°, respectively, with respectto an electric vector transmission direction of the output polariser.30. A display device according to claim 28, wherein the at least onepassive retarder further comprises an additional pair of passiveretarders disposed between the first-mentioned pair of passive retardersand which have optical axes in the plane of the passive retarders thatare crossed.
 31. A display device according to claim 30, wherein theadditional pair of passive retarders have optical axes that each extendat 0° and at 90°, respectively, with respect to an electric vectortransmission direction that is parallel to the electric vectortransmission of the output polariser.
 32. A display device according toclaim 25, wherein the at least one passive retarder comprise a passiveretarder having an optical axis that is oriented with a componentperpendicular to the plane of the retarder and a component in the planeof the retarder.
 33. A display device according to claim 32, wherein thecomponent in the plane of the passive retarder extends at 0°, withrespect to an electric vector transmission direction that is parallel orperpendicular to the electric vector transmission of the displaypolariser.
 34. A display device according to claim 32, wherein the atleast one passive retarder further comprise a passive retarder having anoptical axis perpendicular to the plane of the passive retarder or apair of passive retarders which have optical axes in the plane of thepassive retarders that are crossed.
 35. A display device according toclaim 1, further comprising at least one further polar control retarderarranged between the output polariser and the reflective polariser. 36.A display device according to claim 35, wherein a further additionalpolariser is arranged between the at least one further polar controlretarder and the reflective polariser.
 37. A display device according toclaim 35, wherein the first-mentioned at least one polar controlretarder comprises a first switchable liquid crystal retarder comprisinga first layer of liquid crystal material, and the at least one furtherpolar control retarder comprises a second switchable liquid crystalretarder comprising a second layer of liquid crystal material.
 38. Adisplay device according to claim 37, wherein the first and secondliquid crystal retarders have retardances that are different.
 39. Adisplay device according to claim 38, further comprising a controlsystem arranged to apply a common voltage across the first and secondswitchable liquid crystal retarders, and wherein the liquid crystalmaterial of the first liquid crystal retarder is different from theliquid crystal material of the second liquid crystal retarder.
 40. Adisplay device according to claim 37, wherein the layers of liquidcrystal material of each of the first and second switchable liquidcrystal retarders have a retardance for light of a wavelength of 550 nmin a range from 450 nm to 850 nm, preferably in a range from 500 nm to750 nm and most preferably in a range from 550 nm to 650 nm.
 41. Adisplay device according to claim 37, wherein: the first-mentioned atleast one polar control retarder further comprises a pair of passiveretarders which have optical axes in the plane of the retarders that arecrossed and extend at 45° and 135°, respectively, with respect to anelectric vector transmission direction of the output polariser; the atleast one further polar control retarder comprises a further pair ofpassive retarders which have optical axes in the plane of the retardersthat are crossed and extend at 45° and 135°, respectively with respectto an electric vector transmission direction of the output polariser;and the optical axes of the one of the first-mentioned pair of passiveretarders and the one of the further pair of passive retarders that areclosest to each other extend in the same direction.
 42. A display deviceaccording to claim 41, wherein each passive retarder of thefirst-mentioned pair of passive retarders, and each passive retarder ofthe further pair of passive retarders, has a retardance for light of awavelength of 550 nm in a range from 300 nm to 800 nm, preferably in arange from 350 nm to 650 nm and most preferably in a range from 400 nmto 550 nm.
 43. A display device according to claim 35, wherein the atleast one further polar control retarder comprises at least one furtherpassive retarder.
 44. A display device according to claim 1, furthercomprising: a backlight arranged to output light, wherein the SLM is atransmissive SLM arranged to receive output light from the backlight,and the SLM further comprises an input polariser arranged on the inputside of the SLM, the input polariser being a linear polariser; and afurther additional polariser arranged on the input side of the inputpolariser, the further additional polariser being a linear polariser;and at least one further polar control retarder arranged between thefurther additional polariser and the input polariser.
 45. A displaydevice according to claim 44, wherein the first-mentioned at least onepolar control retarder comprises a first switchable liquid crystalretarder comprising a first layer of liquid crystal material, and the atleast one further polar control retarder comprises a second switchableliquid crystal retarder comprising a second layer of liquid crystalmaterial.
 46. A display device according to claim 45, wherein the firstand second liquid crystal retarders have retardances that are different.47. A display device according to claim 46, further comprising a controlsystem arranged to apply a common voltage across the first and secondswitchable liquid crystal retarders, and wherein the liquid crystalmaterial of the first liquid crystal retarder is different from theliquid crystal material of the second liquid crystal retarder.
 48. Adisplay device according to claim 45, wherein the layers of liquidcrystal material of each of the first and second switchable liquidcrystal retarders have a retardance for light of a wavelength of 550 nmin a range from 450 nm to 850 nm, preferably in a range from 500 nm to750 nm and most preferably in a range from 550 nm to 650 nm.
 49. Adisplay device according to claim 45, wherein: the first-mentioned atleast one polar control retarder further comprises a pair of passiveretarders which have optical axes in the plane of the retarders that arecrossed and extend at 45° and 135°, respectively, with respect to anelectric vector transmission direction of the output polariser; the atleast one further polar control retarder comprises a further pair ofpassive retarders which have optical axes in the plane of the retardersthat are crossed and extend at 45° and 135°, respectively with respectto an electric vector transmission direction of the output polariser;and the optical axes of the one of the first-mentioned pair of passiveretarders and the one of the further pair of passive retarders that areclosest to each other extend in the same direction.
 50. A display deviceaccording to claim 49, wherein each passive retarder of thefirst-mentioned pair of passive retarders, and each passive retarder ofthe further pair of passive retarders, has a retardance for light of awavelength of 550 nm in a range from 300 nm to 800 nm, preferably in arange from 350 nm to 650 nm and most preferably in a range from 400 nmto 550 nm.
 51. A display device according to claim 44, wherein the atleast one further polar control retarder comprises at least one furtherpassive retarder.
 52. A display device according to claim 1, furthercomprising: a backlight arranged to output light, the SLM being atransmissive SLM arranged to receive output light from the backlight,wherein the backlight provides a luminance at polar angles to the normalto the SLM greater than 45 degrees that is at most 30% of the luminancealong the normal to the SLM, preferably at most 20% of the luminancealong the normal to the SLM, and most preferably at most 10% of theluminance along the normal to the SLM.
 53. A display device according toclaim 1, wherein the SLM is an emissive SLM and the display polariser isan output polariser arranged on the output side of the SLM
 54. A displaydevice according to claim 1, wherein the reflective polariser and theoutput polariser have electric vector transmission directions that areparallel.
 55. A display device according to claim 1, wherein thereflective polariser and the additional polariser have electric vectortransmission directions that are parallel.
 56. A display deviceaccording to claim 1, wherein the reflective polariser and theadditional polariser have electric vector transmission directions thatare not parallel, and the display device further comprises a rotatorretarder arranged between the reflective polariser and the additionalpolariser, the rotator retarder being arranged to rotate a polarisationdirection of polarised light incident thereon between the electricvector transmission directions of the display polariser and theadditional polariser.
 57. A view angle control optical element forapplication to the output side of a display device for use in ambientillumination comprising a spatial light modulator (SLM) arranged tooutput light; wherein the SLM comprises an output polariser arranged onthe output side of the SLM; the view angle control optical elementcomprising an additional polariser; a reflective polariser arrangedbetween the output polariser and the additional polariser on applicationof the view angle control optical element to the display device; and atleast one polar control retarder arranged between the reflectivepolariser and the additional polariser, wherein the at least one polarcontrol retarder is capable of simultaneously introducing no netrelative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser along an axis along a normal to theplane of the at least one polar control retarder and introducing arelative phase shift to orthogonal polarisation components of lightpassed by the reflective polariser along an axis inclined to a normal tothe plane of the at least one polar control retarder.
 58. A displaydevice comprising: a spatial light modulator (SLM); a display polariserarranged on at least one side of the SLM, the display polariser being alinear polariser; a first additional polariser arranged on the same sideof the SLM as one of the at least one display polarisers, the firstadditional polariser being a linear polariser; and first polar controlretarders arranged between the first additional polariser and the one ofthe at least one display polarisers; a further additional polariserarranged on the same side of the SLM as said one of the at least onedisplay polarisers, outside the first additional polariser, the furtheradditional polariser being a linear polariser; and further polar controlretarders arranged between the first additional polariser and thefurther additional polariser, wherein the first polar control retarderscomprise a pair of passive retarders which have optical axes in theplane of the retarders that are crossed and extend at 45° and 135°,respectively, with respect to an electric vector transmission directionof the output polariser, the further polar control retarders comprise afurther pair of passive retarders which have optical axes in the planeof the retarders that are crossed and extend at 45° and 135°,respectively with respect to an electric vector transmission directionof the output polariser, and the optical axes of the one of the firstpair of passive polar control retarders and the one of the further pairof passive polar control retarders that are closest to each other extendin the same direction.
 59. A display device according to claim 58,wherein the first polar control retarders and further polar controlretarders each further comprise a switchable liquid crystal retardercomprising a layer of liquid crystal material, the first polar controlretarders and the further polar control retarders each being arranged,in a switchable state of the switchable liquid crystal retarder,simultaneously to introduce no net relative phase shift to orthogonalpolarisation components of light passed by said one of the at least onedisplay polarisers along an axis along a normal to the plane of thepolar control retarders and to introduce a net relative phase shift toorthogonal polarisation components of light passed by said one of the atleast one display polarisers along an axis inclined to a normal to theplane of the polar control retarders.
 60. A display device according toclaim 58, further comprising a backlight arranged to output light, theSLM being a transmissive SLM arranged to receive output light from thebacklight, and the display device comprising a display polariser that isan input polariser arranged on the input side of the SLM and an outputpolariser arranged on the output side of the SLM.
 61. A display deviceaccording to claim 58, wherein the SLM is an emissive SLM and thedisplay polariser is an output polariser arranged on the output side ofthe SLM.
 62. A display device comprising: a backlight arranged to outputlight; a transmissive spatial light modulator (SLM) arranged to receiveoutput light from the backlight; an input polariser arranged on theinput side of the SLM and an output polariser arranged on the outputside of the SLM, the input polariser and the output polariser beinglinear polarisers; a first additional polariser arranged on the outputside of output polariser, the first additional polariser being a linearpolariser; and first polar control retarders arranged between the firstadditional polariser and the output polariser; a further additionalpolariser arranged between the backlight and input polariser, thefurther additional polariser being a linear polariser; and further polarcontrol retarders arranged between the first additional polariser andthe input polariser; wherein the first polar control retarders comprisea pair of passive retarders which have optical axes in the plane of theretarders that are crossed and extend at 45° and 135°, respectively,with respect to an electric vector transmission direction of the outputpolariser, the further polar control retarders comprise a further pairof passive retarders which have optical axes in the plane of theretarders that are crossed and extend at 45° and 135°, respectively withrespect to an electric vector transmission direction of the outputpolariser, and the optical axes of the one of the first pair of passivepolar control retarders and the one of the further pair of passive polarcontrol retarders that are closest to each other extend in the samedirection.
 63. A display device according to claim 62, wherein the firstpolar control retarders and further polar control retarders each furthercomprise a switchable liquid crystal retarder comprising a layer ofliquid crystal material, the first polar control retarders and thefurther polar control retarders each being arranged, in a switchablestate of the switchable liquid crystal retarder, simultaneously tointroduce no net relative phase shift to orthogonal polarisationcomponents of light passed by said one of the at least one displaypolarisers along an axis along a normal to the plane of the polarcontrol retarders and to introduce a net relative phase shift toorthogonal polarisation components of light passed by said one of the atleast one display polarisers along an axis inclined to a normal to theplane of the polar control retarders.